Prepared by:
Steven L. Johnson, Ph.D., PE, CPE and Naveen Pawar
Department of Industrial Engineering
Funded by:
Mack-Blackwell Rural
November
2005
The researchers would like to
express sincere appreciation to the participating companies that provided
valuable advice and guidance during the conduct of this effort. Although many
individuals provided important input, the authors would like to specifically
thank Grant DuCote, Greer Woodruff and Henry Pianalto of JB Hunt Transport
Services; Shannon Lively, Jim McFarland and Mark Bradley of ABF Freight
Systems; and Rick Foster and Mark Helms
of Wal-Mart. We would also like to thank the many contributors from other
trucking organizations and the original equipment manufacturers that provided
valuable information that was not available from public sources. Lastly, we
would like to acknowledge the many truck drivers who offered their time and
opinions at the truck stops and rest areas. This research would not have been
possible without the contributions of the stakeholders.
1. Introduction
2. Review and Analysis of Literature
2.1
Setting Speed Limits Based on the 85th Percentile ……….……………………
5
2.2
Effects of Speed Limits on the Distribution of Traffic Speed ..
2.2.1 Effects of Posted Limits on Mean Speed and Speed Variance…
2.3
Effects of Speed Limits on Rural
2.3.1 General Trends in Highway Safety………………………………...…… 15
2.3.2 Methodological Issues Contributing to
Different Study Results .….... 17
2.3.3 Cause and Impact of Speed Variation …….
2.3.4 Effects of Speed on Individual Vehicle
Risk…..
2.3.5 Effects of Speed on Crash Severity ...………………………………….. 28
2.3.6 International Studies of the Safety Impact
of Speed Limits
2.3.7 Studies of Speed Limit Changes in the
2.3.7.1 Studies Prior to 1987 …………………………….…………..…. . 37
2.3.7.2 Impact of the 1987 Speed Limit Increase….………………….. . 38
2.3.7.3 Impact of the 1995 Speed Limit Increase
…………………….. . 45
2.3.8 The Effect of Differential Speed Limits on
Safety .…1
2.3.9 Cause and Impact of Truck Accidents
2.4
Effects of Speed on Driver Fatigue
2.5
Effects of Speed and Weight on Braking Distance……………………….…... 63
2.6
Effects of Speed on Operational Costs………………………….…………….... 65
2.6.1 Effects of Speed on Fuel Costs
2.6.2 Effects of Speed on Tire Costs
2.6.3 Effects of Speed on Maintenance Costs
2.7 Effects of Speed on Pollution
3. Research Methodology……………………………………………………….……..…... . 75
3.1
Measurement of Traffic Speeds on Highways with Different Limits……..….. 7
3.2 Computer Simulation Evaluation of Speed Differentials on Interactions
3.3 Assessment of Speed Limiters Use on Heavy
Trucks
3.4
Survey of Truck Drivers’ Opinions
3.5
Survey of Carrier Fleet Safety and Maintenance Personnel
3.6
Survey of Equipment Manufacturers of Trucks, Engines and Tires
3.7
Comparison of Fleet Experience in States with Different Speed Limits…..... 78
3.8 Financial Cost-Benefit Analysis of Operating Speeds ……………..……..….. 79
4. Analyses and Results…………………………………………………………………..…. 80
4.1 Traffic
Speed Measurement under Different Speed Limits Configurations… . 80
4.1.1
4.1.2
4.1.3
4.1.4
4.1.5 Summary of Speed Data from Different
Speed Configurations…….. 86
4.1.5.1 Speed Differentials and Compliance
4.1.5.2 Posted Speed Limits and Mean Speeds and
Differentials….. . 89
4.1.5.3 Posted Speed Limits and Speed Variance
4.1.5.4 Speed Differentials and Clustered Congestion……………….. 93
4.2
Impact of Speed Differentials on the Number of Vehicle Interactions…….…. 95
4.3
Speed Limiter Use on Heavy Trucks
4.3.1 Driver Category and Speed Limiter Settings………………………….1 00
4.3.2 Distribution of Speed
Limiter Setting……………………………….….101
4.3.3 Driver Experience and Speed
Limiter Setting………………………... 101
4.3.4 Fleet Size and Speed Limiter
Setting…………………………………. 101
4.4
Opinions of Truck Drivers
4.4.1 Characteristics of Vehicles and
Routes…………………….………… 103
4.4.2 Truck Driver’s Opinion of
the Effects of Vehicle Interactions……….105
4.4.2 Effects of Vehicle Interactions …………………………………..……. 104
4.4.3 Effects of Speed Differentials at On-Ramps
and Off-Ramps………. 108
4.4.4 Effects of Speed and Speed
Differentials on Driver Fatigue……….. 109
4.4.5 Effects of Speed Limits on Driver
Retention……………………..….. 110
4.4.6 Effects of Speed and Speed Differentials on Operating
Costs…….. 110
4.4.7 Comparison of Owner-Operator and Company
Driver Opinions…... 111
4.5 Opinions
of Carrier Fleet Safety and Maintenance Management………..…. 114
4.6 Opinions
of Original Equipment Manufacturers…………………………….........
4.6.1 Opinions of Engine Manufacturers
4.6.2 Opinions of Tire Manufacturers
4.7
Comparison of Fleet Experience in States with Different Speed Limits
4.7.1 Selection of Accident Data
4.7.2 Analyzing Accident Data by State Speed
Limits.
4.8
Financial Cost-Benefit Analysis of Operating Speeds……..........................
......
5. Summary………….……………………………………………………………………….. 1
5.1
Research on Truck Speed Effects
on Traffic Flow and Safety………..…..... 122
5.1.1 Impact of Speed Limits on Traffic Speed
…………………………..... 122
5.1.2 Impact of Speed Limits on Rural
5.1.3 Causes and Impact of Speed Variance
……………………………… 124
5.1.4 Impact of Speed on Crash Severity
…………………………….…..... 126
5.1.5 Impact of Differential Speed Limits on
Highway Safety …….……..... 126
5.1.6 Effect of Speed on Driver Fatigue
………………….……………….... 128
5.2
Effect of Speed on Operation Costs ………………………………………..... 128
5.2.1 Effect of Speed on Fuel Efficiency
………..………………………….. 128
5.2.2 Effect of Speed on Roadway Wear
………………………………….. 129
5.2.3 Effect of Speed on Tire Costs
………………………………………... 129
5.2.4 Effect of Speed on Engine Life and Routine
Maintenance Costs ..... 130
5.3
Financial Cost-Benefit Analysis of Operating Speeds …………………….... 130
5.4
Conclusions …………………………………………………………….………. 131
6. References …………………………………………………………………………...... 132
Appendices………………………………………………………………………….………. 146
A. Speed
Limits Before 55 mph NMSL in 1974 (Source: Atkinson, 1996)…...........
B. 1987
Speed Limit Increase ……………………………………………….............
C. 1995
Speed Limit Increase ……………………………………..………..….........
D. Rural
Interstate Speed Limits ….........................................................................
E. Summary
of Speed Data at Individual Sites……………………………...….. 150
F. Truck
Driver Survey ……………….………………………………...………... 152
G. Safety
Manager Survey ……………….…………………………...………..... 154
H. Maintenance
Manager Survey………………..……………...……………...... 156
I.
Survey
Statistics ……………………………………………………………….. 158
1.
Introduction
The setting of speed
limits has been controversial since the first speed limits were set in 1901. Other
than during the period of the National Maximum Speed Limit policy between 1973
and 1994, setting speed limits has historically been the responsibility of the
states. Posted speed limits on
In addition to
state-regulated maximum speed limits, traffic flow is affected by the fact that
most commercial truck fleets and many owner-operators have speed limiters on
their vehicles. These limiters result in speed differentials between many
trucks and automobiles, even if the posted limits are not different. The primary
reasons that trucking companies use speed limiters include safety and a
reduction in operating costs associated with fuel efficiency. The potential financial
benefits of increasing per-truck revenues versus the additional costs
associated with higher speeds are discussed. The objective is to provide
information for both regulatory agencies and commercial trucking operations in
the decision process of setting maximum truck speeds on rural interstate
highways.
The initial portion
of the report reviews the research and applications literature related to the
factors that are affected by vehicle speed. The empirical studies that have
addressed the effect of changes in highway speed limits on traffic flow and the
distribution of vehicle speeds are discussed. Understanding the causes of
highway accidents that involve trucks is important in order to evaluate the
effect of speed on highway safety. The causes of single and multiple vehicle
accidents involving trucks were reviewed. The extensive literature that has
dealt with the safety impact of increasing and decreasing speed limits at both
the national and state levels is critically reviewed. In particular, the
results of safety studies after the 1974 decrease in national speed limits to
55 mph and the subsequent increases in 1987 and 1995 are evaluated. The
methodological issues that help explain the many different conclusions drawn
from this body of research are presented.
The effects of both
absolute speed and differential speed limits are discussed in the context of
traffic flow and speed variation. Whether being due to state regulated limits
or company policies, the difference in speed between heavy trucks and
automobiles results in more speed variance. The research literature that
discusses the impact of speed variance on highway safety is presented.
There is a
relationship between vehicle speed and the amount of time required to cover a
particular distance. This is important for motorists, although it is
particularly important for commercial transport operations. The effect of
driving time has been an issue that has received a significant amount of
attention from the trucking industry, governmental agencies, and the general
public in the context of truck driver “hours of service.” The research
literature that addresses the effect of driving time and driving speed is
discussed with respect to driver fatigue.
In addition to the
safety implications, the operational costs associated with truck speeds are
important in a benefit/cost analysis. The research and applications literature
that pertains to the costs of direct costs such as fuel, tires, and maintenance
are discussed. In addition, the research that addresses the indirect costs such
as emissions and road wear are presented.
The next portion of
this research effort collected data in an attempt to fill some of the holes
that were observed in the literature. For example, although there was a very
large amount of research on speed limits, virtually none of the studies had
recognized the impact of speed limiters on heavy commercial trucks. Even the
studies that specifically analyzed increases in traffic speed when posted
limits were increased (e.g., 1987 and 1995) did not account for the fact that
the majority of heavy trucks, which often make up a large portion of the
traffic on interstate highways, could not increase their speed.
To address this
issue, empirical data were collected under four different speed limit
configurations. Data were collected on Interstate I-44 where the speed limit is
70 mph for both automobiles and trucks. The Cherokee Turnpike in
The empirical distributions
for truck and automobile speeds that were observed at two of the locations (
As previously
stated, even in states that have uniform speed limits on the rural interstates,
there is still a difference in the speeds of automobiles and heavy trucks due
to company speed limitation policies. The next portion of the study collected
data on the use of speed limiters by commercial trucking operations. The data
were collected from 236 drivers at truck stops in seven different states (AR,
IL, MO, OK, NM, AZ, TX). These drivers represented the full spectrum of
owner-operators operating under their own authority, lease/contract drivers,
and employees of large trucking fleets. The distribution of settings used by
these different groups provides an important reference point for understanding
how truck speed limiters affect traffic flow under different speed limit
configurations.
Surveys were
completed by the 236 drivers that addressed their opinions about speed limits
on rural interstates and speed differentials between automobiles and heavy
trucks. The surveys addressed perceived safety issues, as well as the drivers’
judgments about the effect of truck speed on operational costs (fuel, tires,
etc.) and psychosocial factors (driver fatigue, stress, and driver retention). The
specific effects on drivers of speed differentials, whether due to posted limits
or company policies, were documented.
In addition to
collecting opinion data from the drivers, the opinion of commercial fleet
management personnel were obtained through a combination of surveys, on-site
visits, and communications at professional and trade meetings. In particular,
the opinions of fleet safety and maintenance managers were collected, along
with any data that the fleets had that pertained to the effects of truck speed
and speed differentials. These opinions were then contrasted with the information
that was obtained from the literature review and the opinions of the truck
drivers.
The last group surveyed represented the
original equipment manufacturers of the components that could be affected by
the vehicle speed. In particular,
manufacturers of commercial trucks, engines, and tires were surveyed with
respect to the effect of truck speed on their products. These communications included both technical
sales personnel and engineers in the various companies’ technical and research
centers.
Data from
participating companies were used to conduct an analysis of “virtual”
differential speed limits between automobiles and heavy trucks. The companies
with fixed maximum speeds that were limited to either 62 or 65 mph operated in
different states with different maximum speed limits for automobiles (65, 70 or
75 mph). By comparing the accident data from these different situations, the
impact of a “virtual” speed differential
between the fleets’ trucks and the automobiles was analyzed.
The last section of
the report addresses the financial benefit-cost relationships associated with
higher truck speeds. There is a trade-off between the benefits of increased
company revenue that could be attainable with higher truck speeds and the
increased operational costs incurred at higher truck speeds.
The issue of speed
differentials between automobiles and heavy trucks is a complex combination of
the impact on safety and financial considerations for both the truck drivers
and the commercial trucking organizations. This report addresses the currently
available published information, as well as the opinions of the various
stakeholders with respect to the benefits and costs of limiting heavy truck
speeds to below the traffic speed. This
information is important for both public policy and company policy related to
setting speed limits on rural interstate highways.
The objective of this
effort was to investigate the costs and benefits related to speed differentials
between heavy trucks and other vehicles on rural interstate highways. Truck
speeds are limited by a combination of state regulated speed limits and company
policies that limit truck speed with electronic control units on the trucks’ engines.
Both the effect of absolute speed and the speed of trucks relative to the other
vehicles in the traffic flow are important to understand the impact of heavy
truck speed policies. The initial phase of the effort involved a comprehensive
review of the research and applications literature that pertains to the topic.
The first part of
the literature review addresses the standard methods used to set posted speed
limits and the impact of speed limits on the speed distributions of both heavy
trucks and other traffic. The next section reviews the literature that has
documented how speed limits and speed limit changes affect accident and
fatality rates in the United States and internationally. The extensive number
of studies that have investigated the safety impact of increases and decreases
in speed limits has been reviewed. The last part of that section specifically
addresses the causes and impact of heavy truck accidents and the impact of
speed differentials between trucks and automobiles. The research literature
pertaining to the relationship of vehicle speed and driver fatigue is
discussed.
The last sections
of the literature review address the research and applications literature on
the operational impact of speed. In particular, the effects of truck speed on
fuel consumption, tire costs, and maintenance costs are discussed.
2.1 Setting Speed Limits
Based on the 85th Percentile
The geometric features of the roadway, such as
horizontal and vertical alignment, sight distance, and cross-section determine
the highway design speed. The original definition of design speed, coined by the
American Association of State Highway and Transportation Officials (AASHTO) in
1938, was “the maximum approximately uniform speed which probably will be
adopted by the faster group of drivers but not, necessarily, by the small
percentage of reckless ones” (Krammes, Fitzpatrick, Blaschke, and Fambro,
1996). AASHTO’s current definition of design speed is “the maximum safe speeds
that can be maintained over a specified section of highway when conditions are
so favorable that the design features of the highway govern” (AASHTO,
2001). This is the maximum speed prudent
drivers would choose when environmental conditions are very good and traffic
volumes are low. Subject to the constraints of environmental quality, economics,
aesthetics, and social impacts, AASHTO recommends higher design speeds to
promote safety, mobility, and efficiency.
Design speed is highly sensitive to certain highway design features like
curvature, sight distance, and roadside elements.
When speed limits are set based on design speed, the
posted speed limit is generally lower than the design speed because it is known
that some drivers will tend to drive faster and also that the road conditions
are sometimes poorer than were used in the design standards (Persaud, Parker,
Knowles and Wilde, 1997). However, according to Abraham and Abdulhai (2001), a
speed limit that is set using this as a basis will often appear unrealistic to
drivers since the limit is for an entire highway segment, even though is often
reflects relatively few elements.
According to AASHTO (2001) posted speed limits are
usually set to approximate the 85th percentile speed of traffic. For
many rural highways, it is a common practice to establish the speed limit near
the 85th percentile speed.
The term “85th percentile speed” is the speed at or below
which 85% of drivers travel in free-flow conditions at representative locations
on the highway or roadway section (National Research Council, 1998). The 85th percentile speed is determined
through spot speed studies of “free flowing” traffic (i.e., traffic unimpeded
by other vehicles) (Krammes, Fitzpatrick, Blaschke and Fambro, 1996). According
to AASHTO (2001) the 85th percentile speed is usually within the
“pace” or the 10 mph speed range used by most drivers. In general, the speed
limits for rural interstates are set below the 85th percentile speed
limits. Harkey, Robertson and Davis (1989) collected data from urban and rural
highways in
The 85th percentile speed for a
distribution of speed observations is shown in Figure 1. In most cases, the difference between the
85th percentile speed and the average speed provides a good approximation of speed
standard deviation, which is another important factor that relates to the
speed-safety relationship.
The distribution of
traffic speeds on any particular highway is affected by the posted speed limits
and the enforcement of the limits. The observed 85th percentile
speed on a highway with a 65 mph speed limit will be different than the 85th
percentile speed on a highway with a 75 mph speed limit, even if they are both
rural interstates with identical geometries.
For this reason,
although it is discusses many times in the context of setting speed limits on
rural interstate highways (Governors Highway Safety Association, 2005), the
concept of “design speed as defined by the 85th percentile” does not
appear to make apply. Safety, efficiency,
and economics have played a significant part in the process of setting limits.
This is shown by the large differences in speed limits set on similar highways
in different states.
The first speed
limit in the
The National
Highway Traffic Safety Administration (1992) analyzed the speed data available
from 18 of the 40 states that increased the automobile speed limits from 55 mph
to 65 mph in 1987. The average speed of automobiles increased from 60.4 mph in
1986 to 64 mph in 1990. It was concluded that the increase in the speed limit
significantly increased the average traffic speed. However, another way of looking
at the same statistics is that the average driver’s speed exceeded the posted
speed limit by 5.4 mph in 1986, while in 1990 the average speed was actually 1
mph below the posted speed limit.
Freedman and
Esterlitz (1990) measured the effect of increased speed limits on the traffic
speed in Virginia and found that a 10 mph increase for automobiles speed limit,
from 55 mph to 65 mph (leaving truck limits at 55 mph), resulted in an increase
in the average speed of automobiles of 2.8 mph (63.1 to 65.9 mph) within one
month of implementation. Later, as drivers “adapted” to the new speed limits,
the average speed gradually increased, reaching 66.9 mph after one year. The
authors contended that the percentage of automobiles “over speeding” (traveling
above 65 mph) doubled from 32% to 69%. Again, however, another way of
presenting the statistics is that the compliance rate increased and the average
speed was reduced from 8 mph above the speed limit to only 0.9 mph above the
speed limit. The conclusion as to the
effect of a speed limit on “speeding” depends upon the definition. The average speed observed in this study was
significantly lower than the average speed observed by the National Highway
Traffic Safety Administration (1992) because the former study considered only
the automobile speed, whereas the latter study included heavy trucks.
Godwin (1992) found
that an increase of 10 mph (55 mph to 65 mph) increased the average traffic
speed by 3 mph (60.2 mph in 1986 to 63.2 mph in 1988). In the same period, the
average speed in states that maintained the 55 mph speed limit increased by 0.9
mph (58.7 mph to 59.6 mph). Nakao (1989)
found similar results for automobile speed data. A 10 mph speed limit increase (55 mph to 65
mph) resulted in 2.5 mph increase in average speed (62.4 mph to 64.9 mph) from
April 1987 to September 1987. However,
the observed speed change might have been greater if the data were collected later,
when the drivers had “adapted” to the new speed limits. Any increase in the speed
limit is followed by a “transition” period and then by “adaptation.” During the
initial “transition” period, the drivers’ speed does not increase suddenly to
the new higher speed, although it does increase gradually. After the transition period, they become
“adapted” to the new higher speed limits and travel at the higher speeds. Ledolter and Chan (1996) found that after the
1987 increase in the speed limit in
McKnight, Klein,
and Tippetts, (1989) analyzed nationwide data from 1983-1988 and found that the
number of drivers spotted “speeding” increased by 48% for the states which had
increased their maximum speed limit to 65 mph; whereas the number of drivers
observed “speeding” increased by only 18% in the states that retained the 55
mph maximum speed limit. However, an important point to be noted is that the
definition of “speeding” in this study was “anyone traveling at speeds higher
than 65 mph” Obviously the number of people traveling above 65 mph in a 65 mph
speed limit state will be much higher compared to the number in a state with a 55
mph speed limit. It will be observed that in many of the studies discussed, the
researchers defined speeding as the percentage of drivers who exceeded 65 mph
because it is widely assumed that high speeds are the primary contributors to
fatal accidents. This definition of speeding does not consider the design speed
of the highways, which is a major factor in determining the effects of traffic
speed. Many of the highways included in
the studies have design speeds that far exceed 65 mph.
Agent, Pigman, and Webber
(1998) conducted a study to evaluate the effect of speed limits in
Similar results
were obtained by Parker (1992); however, his study was limited to only rural
and urban highways that were not limited access. Parker collected speed and
accident data from 100 sites in 22 states before and after speed limits were
altered. The average change in any of
the percentile speeds (i.e., 90th, 80th, etc.) at the
experimental sites was less than 1.5 mph, regardless of whether the speed limit
was raised or lowered. This indicates that
distribution of speed remains relatively constant and that the average speed of
traffic generally follows an increasing trend, irrespective of the change in
posted speed limits. The authors concluded that speed limits that are set close
to the 85th percentile speed had a beneficial effect on the drivers’ tendency
to comply with the posted speed limits. It was concluded that lowering and raising
the speed limits has relatively little effect on the traffic speed and that drivers
travel according to the traffic conditions.
Binkowski, Maleck,
Taylor, and Czewski (1998) studied the 1996 increase in speed limits for automobiles
from 65 mph to 70 mph in
Najjar, Stokes, Russell, Ali, and Zhang (2000) studied the results of the
1996 increase in maximum speed limit from 65 mph to 70 mph in
Borsje (1995)
studied the effects of having different speed limits on different highways within
the same highway category (referred to by the authors as differentiated speed
limits) in the
In addition to the
direct impact on traffic speed resulting from increases in posted limits on
highways, there are two indirect effects on traffic: speed spillover and
traffic diversion. Speed spillover results
when an increase in the speed limit on one highway increases the average
traffic speed on other highways that have not had an increased limit.
McKnight and Klein
(1990) studied the nationwide impact of increasing the speed limits on rural
interstate highways to 65 mph. It was found that for the states that raised
speed limits to 65 mph, speeding on rural interstates and on non-rural interstates
(highways still posted at 55 mph) increased by 48% and 9%, respectively.
Whereas, in states that maintained the 55 mph limit on rural interstates,
speeding increased by 18% and 37% on rural interstates and non-rural
interstates, respectively. It is important to note that “speeding” was defined
as the percentage of drivers who exceeded 65 mph for both the 55 mph and 65 mph
highways.
Nakao (1989)
analyzed the 1987 speed data from
Mace and Heckard
(1991) collected data between 1986 and 1988 from Illinois, Ohio, Texas and
Alabama and found that the average traffic speed for states that increased
their speed limit from 55 mph to 65 mph increased by 4 mph; whereas, on roads
still having a 55 mph posted speed limit in these states, the average speed
increased by only 0.8 mph. This study does not support a spillover effect.
A “traffic
diversion” effect occurs when an increase in the speed limits on certain
highways leads to an increase in traffic on the interstates that have a higher
speed limit and a reduction of traffic on highways with lower speed limits.
Lave and Elias (1994) observed the national traffic volumes before and after
the 1987 speed limit increase. They observed that there was a 73% greater increase
in vehicle miles traveled on the higher speed interstates compared to the
statewide value. The non-interstate vehicle miles traveled decreased by 11%. These
values illustrate that the speed limit increase resulted in traffic shifting from
lower speed limit roads to higher speed limit roads.
Comparing the
results of these studies indicates that the increase in speed limits does
appear to increase the average speed and the 85th percentile.
However, the magnitude of these increases has been found to vary significantly in
different studies. One of the reasons for the differences is the time duration
over which the studies were conducted. For example, the increase in average
traffic speed observed by Ledolter and Chan (1996) was much higher than the
increase observed by Nakao (1989). On possible reason for this difference is
that Nakao took only six months of data into consideration (during the
“transition” period), while Ledolter and Chan measured the speed increases over
10 years (when the drivers had adapted to the higher speed limits). Other factors,
such as the geography of different states, that affects the highway design
speeds and traffic volumes could account for the large differences in results
of the different studies. Borsje (1995) and Davis (1998) concluded that
enforcement can have an even greater effect on traffic speed than the posted
limits. The level from which the speed limit increased, whether it was raised
from 55 mph to 65 mph or from 65 mph to 75 mph, also caused differences in the magnitude
of increases observed by the different studies.
One very important factor
that most of the researchers failed to address, and may not even have realized,
is that the speed of most of the commercial heavy trucks are restricted to
below the posted speed limits by speed limiters, due to company policies. This obviously had a large effect on the
magnitude of traffic speed increases when posted speed limits were raised,
particularly for highways that have a relatively high proportion of heavy
trucks.
As previously
discussed, speed differentials between automobiles and heavy trucks occur due
to two primary factors. First, many states impose lower posted speed limits on
heavy trucks. These regulatory differentials range from 5 mph to 15 mph. The
second factor that results in speed differentials between automobile and heavy
trucks is the speed policy that is employed by commercial trucking companies.
Many companies use speed limiters on the truck engines to restrict the maximum
speed. These devices are becoming increasingly sophisticated in both their
ability to control speed and record the speed that is driven. The literature
discussed in this section relates to the effect of posted speed limits in that
there is virtually no literature that addresses the effect of company speed
policies on traffic speed in general, or truck highway speed, in particular. The
notation will characterize speed limits in the format: 70/65 for differential
limits of 70 mph for automobiles and 65 mph for trucks.
Mace and Heckard
(1991) collected data between 1986 and 1988 in Illinois, Ohio, Texas and
Alabama and found that the automobile speeds were 3.5 mph faster than truck
speeds on interstates with a uniform 65 mph speed limit; whereas automobile
speeds were 6 mph more than truck speeds on interstates with different speed
limits of 65 mph for automobiles and 55 mph for trucks. Therefore, a 10 mph speed differential
resulted in a change of 2.5 mph in the average speed difference between automobiles
and trucks.
Baum, Esterlitz, Zador
and Penny (1991) collected data in
Pfeffer, Stenzel,
and Lee (1991) conducted a time series analysis to study the impact of
differential speed limits for automobiles and trucks in
In 1994, Harkey and
Mera examined the impact of differential speed limit on average speed based on
data from 11 states, all having the same speed limit for automobiles but
different limits for trucks. The states were divided into three groups based on
their speed limits: 65/65, 65/60 and 65/55 mph. The mean speeds for automobiles
under these limits were 67.6, 67.8 and 67.4 mph, respectively, which were not statistically
different. However, the average truck speeds in these states were 63.8, 63.6
and 61.1 mph, respectively, for the 65, 60 and 55 mph truck limits. The average
truck speed in 65/55 states was significantly less than for the 65/65 and 65/60
mph states. According to this study, a speed differential of 5 mph (from 60 to
65) did not have a significant impact on the trucks’ speed and a 10 mph speed
differential decreased truck speed less than 3 mph. Furthermore, the percentage
of automobiles traveling above the speed limit by more than 10 mph was
significantly lower in the 65/55 mph (63.8%) states compared to the 65/65 mph
and 65/60 mph states (68.7 and 66.6% respectively). Even though the automobile speed
limit was uniform across all the states, it appears that the slower trucks in
the 65/55 mph speed limit states had the effect of reducing the average speed
of the automobiles. The non-compliance rate for trucks was much larger in the
65/55 and 65/60 speed limit group (89.4 and 76.5%, respectively) compared to
that in 65/65 group (35.6%).
Garber and Gadiraju
(1991) conducted a study in which they increased the speed limits from 55/55 to
65/55 on test sites and retained the uniform 55 mph speed limit on control
sites in
In the
Freedman and
Williams (1992) collected data from 11 northeastern states to estimate the
effect of differential speed limits on the mean speeds and 85th
percentile speeds. Six of these states had remained at 55/55 mph, three had
increased to 65/65 mph and two employed differential speed limits of 65/55 mph.
It was found that the average speed of automobiles in the states with 65 mph
speed limit was 2 to 5 mph faster than those with 55 mph limits. For trucks, the
mean speeds were 3 to 7 mph faster in states with a 65 mph speed limit than in
states with 55 mph limits. The results indicated that the average truck speed was
more sensitive to the posted speed limit than was the average automobile speed.
This could have been due to the fact that the compliance rate of trucks was higher
than the compliance rate of automobiles. For automobiles, there was no
significant difference in the average speed or the 85th percentile
speed in the 65/55 mph speed limit states (67.7 and 72.2 mph) compared to the
65/65 mph speed limit states (66.7 and 72.1 mph). However, the average and the
85th percentile automobile speeds for the 55/55 mph states were
significantly lower (63.0 and 68.7 mph). The results indicated that the lower
truck speed in differential speed limit states did not have any significant
effect on the average speed of automobiles. The mean and the 85th
percentile speeds of trucks were also not significantly different for states
with 65/55 mph speed limit (61.6 and 66.3 mph, respectively) compared to those
for the 55/55 mph limit states (60.2 and 65.3 mph, respectively). However, the
mean and the 85th percentile truck speed for the 65/65 speed limit
states were significantly higher (65.0 and 69.8 mph). The conclusion was that lower
speed limits for trucks did reduce the average and the 85th
percentile truck speeds. These results were in contrast to the opinions
expressed by Ganote (1997), who believed that a differential speed limit does
not really succeed in lowering truck speeds because the drivers takes into
account the prevailing road conditions.
Most of the studies
have concluded that a 10 mph posted speed differential does not produce a 10
mph difference in the average speed of the automobiles and trucks. In addition,
even under uniform speed limits, the average speed of trucks is 3 mph to 4 mph
slower than the average speed of the automobiles. It was also observed by
Harkey and Mera (1994) that the average speeds of automobiles and trucks are
similar in 65/65 mph and 65/60 mph states, indicating that a speed differential
of 5 mph does not have any significant impact on the truck speed.
The literature available on impact of speed limits on accidents and
fatalities is reviewed in this section. It has been indicated in literature
that vehicle speed is only one of the factors that affect the probability and
type of accidents. The type of roadway and
the design speed of the highway are also important factors affecting the number
and type of accidents. Preston (1996) studied the accident records of
|
Accident Type |
Rural |
Urban |
|
Rear End |
12.90 % |
50.50 % |
|
Sideswipe |
7.30 % |
17.40 % |
|
Right Angle |
8.40 % |
2.40 % |
|
Head On |
1.50 % |
0.80 % |
|
Ran Off Road |
33.70 % |
17.80 % |
|
Hit Deer |
25.10 % |
0.40 % |
|
Other |
11.10 % |
10.70 % |
Figure 2
illustrates the amount of variation in the number of highway fatalities over
the last 40 years. To evaluate the effect of speed limits on highway safety, it
is important to consider the amount of exposure experienced by drivers in terms
of the vehicle miles traveled. Figure 3 illustrates that, although speed limits
have increased, the fatality rate (fatalities per 100 million miles traveled)
has been consistently improving. This is the result of improved safety
characteristics of both vehicles and roadways.
Figure 2. Trends in
National Fatalities
(Source.
Figure
4 shows both the number of fatalities on rural interstates and the vehicle
miles traveled. The trend in fatalities is upward; however, the trend in
vehicle miles traveled is also increasing.
Figure 5 illustrates that the trend in the fatality rate on rural
interstates was actually improving during that period.
65+ mph 55 mph 65 mph
65+ mph 65 mph 55 mph
Over the past 40
years, the relationship between highway speed limits and safety has received an
extraordinary amount of attention in both the research and popular
literature. There have often been
conflicting conclusions reported in this literature. Some studies have found
positive effects of higher speed limits, some found very negative effects and
many have not found there to be a relationship. There are a number of reasons
for these differences. It is apparent from a cursory review of the literature
that much of the public comment and even a significant amount of the research
is biased by the entities conducting the research. In addition, there are
serious methodological issues that need to be considered when interpreting the
research presented in the following sections.
The first
explanation for the differing results from different studies is simply the
natural variation that affects accident rates.
Figure 6 indicates the amount of variation in the number of fatalities
on rural roads in
The
results of speed limit studies can also be affected by the states or regions
compared. Figure 7 shows data from a
study by Ashenfelter and Greenstone (2004). They documented the fatality rates
for the states that adopted the 65 mph limits versus the states that retained
the 55 mph limits. It is apparent that the states that increased e speed limits
had a higher fatality rate both before and after the speed limit increase.
If the studies compared the two groups
after the change, without correcting for this difference, the results would not
represent the actual effect of the speed limit increases. The time frame that is selected for the analysis can also significantly
affect the interpretation of the research results. Some of the studies compare the before-and-after
accident data to evaluate the effect of the speed limit increase. Notice in Figure 7 that there was a
significant drop in the fatality rate in 1989 for the states that maintained
the
55 mph limit.
Subsequently, in 1990 and 1991 the fatality rate increased. By comparison,
the fatality
rates for the states that increased their speed limits decreased steadily from 1989 to 1992. If the relative rate of each group was used
in the analysis and the study compared 1986 to 1989 the conclusion could have been
that there was a large increase in the relative fatality rates for the states
that increased their limits. However, if the study had compared the data from
1986 to 1991, the conclusion could have been that there was no effect of the
increase in speed limits.
Another aspect of the time frame
issue is the adaptation that occurs when a speed limit is changed. There is
inertia to traffic speed when the limits are changed. The average speed and the
85th percentile speed do not change very much initially. In
particular, when limits are changed, a few drivers will adapt rapidly, moving at
new speed limit or even faster; whereas most drivers will increase their speed
gradually as they become more comfortable with the increased speed. The result
is that there is initially an increase in the speed variance among vehicles.
The negative effect of speed variance is potentially confounded with the effect
of the speed limit increase.
As previously discussed, the amount and severity of enforcement also has
a large effect on traffic speed behavior.
If enforcement was relatively lax when the speed limits were lower and
became more strict with new, higher limits, the actual effect of the change on
traffic speed might be minimal. In this case, the impact of increased “posted”
speed limits might have no effect on traffic behavior and, therefore, accident
rates.
The effect of having highway types with very different design speeds is
also important to the interpretation of the speed limit studies. The current
study is focused on rural interstates. Most of the research combined all
highways, some with low design speeds and others (i.e., rural interstates) with
design speeds that are significantly above the posted speed limits. Even for
the studies that specifically address the speed limits on interstate highways,
most do not differentiate between urban and rural interstates. It is often
difficult to extrapolate the results of these studies to rural interstates, in
particular.
The fact that trucks have limiters that often do not allow them to go
the posted speed limit also has an effect on the interpretation of speed limit
studies. When limits were increased from 65 mph in 1995, many, if not most, of
the commercial heavy trucks on the interstate highways were restricted to a speed
of 62 or 65 mph. As previously
discussed, this is the reason that the average vehicle speed generally
increases much less than the amount of the increase in the posted speed limits.
The volume of heavy trucks on the highway can have an effect on the traffic
speed. This issue has not been addressed in studies that have investigated the
safety impact of speed limits changes.
The archival databases that many studies have used for their analyses
include only fatalities and do not include accidents that do not involve a
fatality. The effectiveness of passive safety systems (i.e., seat belts, air
bags, etc.) have improved the “crash worthiness” of vehicles that are involved
in an accident. The result is that the relationship between fatalities and
total accidents changes as a function of time. This is particularly the case
for speed limit studies. The simple physics of higher speed accidents could
have a proportionately larger impact on fatalities than on the number of
accidents. Studies that only address fatalities
can come to very different conclusions about the safety implications of speed
limits compared to studies that include non-fatal accidents.
The last methodological issue that makes the interpretation and
comparison of studies in this area difficult is the use of the number of fatalities
or accidents, rather than the fatality or accident rates. As previously
discussed, studies that simply look at the number of fatalities or accidents,
without considering the vehicle miles traveled, can come to different
conclusions than those that include vehicle miles traveled. This again is
particularly the case for speed limit studies. There is an inverse relationship
between speed and exposure time on the highway. That is, for a given mileage
driven, a driver (truck or automobile) is exposed to the potential of a
collision longer at lower speed limits.
The objective of this section was to introduce some of the
methodological issues that limit the interpretability of much of the vast
amount of research literature on the relationship between safety and posted speed
limits. In particular, many of these issues make it difficult to extrapolate
the research findings to truck speeds on
rural interstates. As the safety research is reviewed in the following
sections, these methodological issues should be kept in mind.
2.3.3 Cause and Impact of Speed Variation
Although there has
been a debate as to the impact of speed limits on accidents, one aspect on
which most of the research is consistent is that speed variance can have a
significant impact on the probability of accidents. There are four primary
methods of calculating speed variance reported in the literature: (a) the standard
deviation of the individual vehicle speeds, (b) the difference between the 85th
percentile speed and the median speed (50th percentile), (c) the
difference between the 85th percentile speed and the mean speed, and
(d) the difference between the 85th percentile and the 15th
percentile speed. However, for the data analysis section of this report, only
the first two of the above four methods were used to calculate the speed
variance.
It has been widely
acknowledged that an increase in speed variance is often associated with an increase
in the probability of accidents. According to the National Research Council
(1998), the narrower the speed distribution (e.g., less spread between the
average speed and the 85th percentile speed), the greater the safety
benefits.
Garber and Gadiraju
(1988 and 1989) found that the level of safety on any highway is related to the
characteristics of the traffic stream and the geometry of the highway. It was
found that the major factor that affected speed variance was the difference
between the posted speed limit and the design speed of the highway. Speed
variance was observed to be the lowest when the posted speed limit was 6 to 12
mph lower than the design speed of the highway. The accident rates were
observed to increase with increasing speed variance for all classes of roads.
For average speeds up to 70 mph, speed variance decreased with increased
average speed. The authors also concluded that the accident rates on a highway
do not necessarily increase with an increase in average speed.
Lave (1985)
collected nationwide average speed and 85th percentile speed data
for 6 different types of highways (rural and urban interstates, arterials and
collectors) from 48 states for 1981 and 1982. Speed dispersion was calculated
as the difference between 85th percentile speed and the mean speed. Speed,
by itself, was not found to have a significant effect on fatality rates.
However, when using speed variance as the metric, 10 out of 12 road types indicated
a statistically significant positive relationship. This result indicated that it is not absolute
speed, but the speed variance that increases fatality rates. It was also
observed that, speed variance decreased with increases in the average speed. A
series of responses to Lave’s models by Levy and Asch (1989), Fowles and Loeb
(1989) and Synder (1989) confirmed the negative effect of speed variance, but also
suggested that there is also an impact of average speed on fatality rates. One
common potential drawback in all of the above models is that the speed data and
accident data do not belong to the same highway types. The fatality data for
all road types were combined and then used with interstate average speeds in
their models. Therefore, the results must be interpreted with care (Monsere,
Newgard, Dill, Rufolo, Wemple, Bertini and Miliken, C., 2004).
Graber and Gadiraju
(1991) studied the impact of a speed limit increase on speed variance in
Baxter (1999) and
Addis (1999) also held a similar opinion of the relationship between speed and
safety. According to Baxter, accidents will increase only if speed increases
beyond the design speed of the highway; whereas, if the posted speed remains
below the design speed of the highway, there will not be a significant increase
in accidents as speed limit increases. Addis (1999), also stressed, although
with no data to support his claim, that speed variance has a significant effect
on the fatality rate and that speed, alone, has no effect on fatality rate.
Garber and Ehrhart
(2000) conducted a study of traffic speed, traffic flow and geometric characteristics
on the crash rates for
A study conducted
by Rajbhandari and Daniel (2002) examined the effects of increase in speed
limits from 55 mph to 65 mph in New Jersey in 1998. The data were collected
from 1997-2000. The increase in speed limit to 65 mph caused more speed
variance between automobiles and trucks and increased the accidents that involved
trucks by 19% (772 per year to 919 per year). There was also a 27% increase in
total accidents in the same period.
Fitzgerald (1989)
studied the increase in the speed limit of trucks from 80 kph to 90 kph, while
retaining the 100 kph speed limits for automobiles in
Liu (1998) examined
accident data from 1969 -1995 in
Godwin (1992)
studied the impact of a 1987 speed limit increase on the speed variance. The
standard deviation of traffic speed increased by 0.8 mph (6.1 to 6.9 mph) for
the states that retained the 55 mph speed limit. For the states that increased
their speed limits, the standard deviation increased by only 0.2 mph (6 to 6.2
mph). Similar conclusions were drawn by Binkowski, Maleck, Taylor and Czewski
(1998), who studied the 1996 increase in speed limits for automobiles from 65
mph to 70 mph in
Pfeffer, Stenzel
and Lee (1991) conducted a time series analysis to examine the impact of
differential speed limits on speed variance in
Freedman and
Esterlitz (1990) measured the effect of increased speed limits on traffic speed
and found that in Virginia, the 10 mph speed limit increase from 55/55 to 65/55
mph, had no significant effect on the standard deviation (a measure of speed variance)
of automobiles and trucks, even after one year of speed limit change.
To analyze the
impact of the increase in speed limits on the speed distribution of vehicles,
Nakao (1989) compared
Zlatoper (1991)
analyzed nationwide data in 1987 and found average speed, speed variance, and
traffic volume to be directly related to accidents. Other factors, such as
spending on highway police and safety, income levels , inspection laws, and
seat belt laws were found to be inversely related to the number of accidents.
Radwan and
Sinha (1978) studied the effect of the decrease in speed limit from 70 mph to
55 mph on truck crashes in
Agent, Pigman and
Webber (1998) conducted a study to evaluate the impact of increasing speed
limits from 55 mph to 65 mph on rural interstates in
Harkey and Mera (1994) examined the impact of
differential speed limits on traffic speed variance based on an investigation
of speed data from 12 states (26 sites) divided into four different speed limit
groups (65/65, 65/60, 65/55 and 55/55 mph). The variance of truck speeds was
higher than for automobile speeds when the truck speed limit was higher. Due to
the speed limiters on trucks, not all trucks could travel at the higher speeds,
resulting in more speed variance for trucks. They found differences in truck
speed variance for ten of thirteen pair-wise comparisons between uniform and
differential speed limit sites. No significant differences were found in the automobile
speed variances at the sites.
From the studies
reviewed it appears that differential speed limits increased the amount of speed
variance among vehicles because trucks travel at lower speeds than the automobiles.
When considering automobiles and trucks individually, different results were observed.
Increases in the speed limits decreased the speed variance among automobiles. However,
due to the presence of speed limiters on trucks, most of the trucks can not
travel at speeds above 70 mph. Therefore, if the speed limit for trucks is
raised to 75 mph the speed variance among trucks increases. Regarding the
impact of speed variance on traffic safety, most of the studies have agreed
that increases in speed variance increases the probability of accidents.
2.3.4 Effects
of Speed on Individual Vehicle Risk
In the previous
sections, the effect of traffic speed and speed limits on traffic safety was
discussed. This section focuses on the role of an individual vehicle’s speed on
the probability of being involved in an accident. It has been argued that an increase
in speed will increase the probability of accidents if the number of
interactions with other vehicles increases. Similarly, if a vehicle moves
slower than the traffic speed, the number of interactions will also increase.
Solomon (1964) conducted a comprehensive study on crashes and how other
roadway, driver, and vehicle characteristics affect the probability of being
involved in a crash. Approximately 600 miles of rural two-lane and four-lane
highways were studied using a spot speed sampling procedure. Interviews with
290,000 drivers were collected over a two-year time period. The travel speed
prior to the crash was collected from 10,000 crash records, as reported by the police
or by the driver. The estimated travel speeds from the accident records were
compared to the speeds measured at representative sites within each study
section. The comparisons indicated that vehicles involved in crashes were
over-represented in both high and low speed categories within the speed
distribution. The crash involvement rate was represented by a U-shaped curve as
a function of the amount of deviation from the average speed. The
accident-involvement, injury, and property damage rates were found to be
highest at speeds significantly below the average traffic speed. The accident
rates were least at the average traffic speed and increased with increasing
speed above the average traffic speed (Figure 8).
Cirillo (1968) also
conducted a study that addressed speed variation. Two thousand vehicles
involved in daytime crashes on interstate highways were analyzed. The data
represented a U-shaped curve similar to the Solomon data. The analysis took
into consideration only the crashes that involved two or more vehicles (rear
end, same direction sideswipe or angle collisions). Data were collected on
rural and urban section of interstate highways from twenty state highway
departments. The type of collision was controlled since the focus was on how
the differences in speeds of vehicles in the same traffic stream contributed to
crashes. The U-shaped curve obtained by Cirillo is shown Figure 8. According to
the Insurance Institute of Highway Safety (1991), one of the main concerns
regarding the validity of the results obtained by Cirillo is that only two-
vehicle accidents were considered while single vehicle crashes were not
included.
To address the
average speed of sections of highway not directly related to the crash location,
the Research Triangle Institute (1970) used a combination of trained on-scene
crash investigators and a system of automated continuous speed monitoring
sensors embedded in the roadway pavement to measure the speed of crash-involved
vehicles and their traffic speeds at the time and location of the crash. Data
were collected on 114 crashes involving 216 vehicles on state highways in
A study by Munden
(1967) conducted on the rural main roads in the
Fildes and Lee
(1993) studied the issues associated with speed and traffic safety in
Another Australian
study, conducted by Kloeden, studied the relationship between free traveling
speed and the risk of involvement in a casualty crash on rural highways with posted
speed limits of 80 kph or greater. A total of 83 crash cases were investigated.
The representative speed (average control speed) was obtained by measuring the
speeds of 830 control passenger vehicles that matched the 83 crash cases by
location, direction of travel, time of day, and day of week. The risk of
involvement in a casualty crash was found to increase more than exponentially
with increasing speed above the mean traffic speed (see Figure 9). Unlike the
results of the studies by Solomon and Cirillo, the traveling speeds below the
mean traffic speed were associated with a lower risk of being involved in a
casualty crash. The crash risk doubled with each 3 mph increase above the speed
limit. One of the possible reasons for the different results obtained by
Kloeden, compared to Solomon or Cirillo is that Kloeden studied the risk of
involvement in casualty crashes; whereas Solomon and Cirillo studied the risk
of involvement in any crash, irrespective of its severity. As the travel speed
increases, the accident severity increases.
Garber and Ehrhart
(2000) found that, as the mean speed increased, the crash rate decreased
slightly until the mean speed reached the posted speed limit of 65 mph, and
then the rate began to increase. The crash rate also increased as the mean
speed increased beyond the speed limit. It was noted that the changes in crash
rates were not necessarily caused by any one independent factor. The changes
were a result of the combined effects of independent factors like speed,
standard deviation, and traffic flow.
Hauer (1971)
performed theoretical analysis of “overtaking.” The study demonstrated that the
number of vehicle interactions, in terms of passing or being passed, is a
U-shaped curve with a minimum at the median speed. The increased risk of crash
involvement was a result of potential conflicts created when a faster vehicle
passes a slower vehicle. The relative overtaking rates for a vehicle as a
function of deviation from mean speed on a 100-kph road is shown in Figure 9.
Harkey, Robertson
and Davis (1989) studied the relationship between speed and accidents on non-55
mph urban roads in
Coffman, Stuster
and Warren (1998) conducted a literature review of all American and
international research to analyze the relationship between speed and accidents.
It was concluded that the crash risk is lowest near the average speed of
traffic and increases for vehicles traveling much faster or slower than
traffic. Finch, Kompter, Lockwood and Maycook (1994) collected international speed
and accident data and performed a regression analysis to study the relationship
between speed and accidents. Their results indicated that the probability of
being involved in an accident was represented by a U-shaped curve as a function
of speed.
2.3.5 Effects of Speed on Crash Severity
The research
literature presents a clear relationship between vehicle speed and the severity
of injury resulting from a crash, when a crash does occur. In a crash, the basic
physics of motion explains this relationship. A vehicle occupant continues in
motion at the pre-crash speed for a short time after impact, until collision
with another surface within or outside the vehicle occurs and completely halts
the motion of the person (Evans, 1991). Seat belts and airbags provide some
protection; however, greater vehicular speed upon impact usually results in
faster motion of an occupant into the vehicle surroundings and a higher chance
of serious injury or death. The
relationship between travel speed and the severity of injuries sustained in a
crash was examined more than 40 years ago by Solomon (1964) who reported an
increase in crash severity with increasing vehicle speeds on rural roads. After
analyzing 10,000 crashes, Solomon observed that crash severity increased
rapidly at speeds in excess of 60 mph, and that the probability of fatal
injuries increased sharply above 70 mph.
The
impact of vehicle speed on the severity of an accident has been significantly
affected by the improvements in automobile and truck crash worthiness. Passive systems, such as seat belts and air
bags, have decreased the severity of highway accidents. Increasingly, active
safety systems, such as lane departure, collision avoidance, and vehicle
stability systems are improving highway safety for both automobiles and heavy
trucks. The improvements in crash worthiness over time have, to some extent, made
the direct relationship between speed and crash severity more difficult to
interpret.
2.3.6 International Studies of the Safety Impact
of Speed Limits
There has been a significant
amount of international research conducted on the issue of the impact of speed
limits on accidents and fatalities. However, as demonstrated by the wide
disparity in rural speed limits in different countries, there is currently no
consensus on the relationship between speed limits and safety. Table 2
summarizes the maximum speed limit in different countries and the accident and
fatality rates in those countries (Source: International Road Traffic and Accident Database, 2004).
|
|
Fatalities per 100,000 pop. |
Injury accidents |
Fatalities per 100 million vehicle
km |
|
Probability of fatality |
|||
|
Country |
Total |
per 100,000 pop. |
per 100 million vehicle km |
All roads |
Motor-ways |
Speed |
Based on VMT |
Based on pop. |
|
|
8.8 |
|
|
0.9 |
|
110 |
|
|
|
|
11.9 |
537 |
55 |
1.23 |
0.72 |
130 |
1.31 |
2.22 |
|
|
14.5 |
462 |
52 |
1.63 |
0.62 |
120 |
1.19 |
3.14 |
|
|
8.9 |
496 |
50 |
0.9 |
0 |
113 |
0.00 |
1.79 |
|
Czech Rep. |
14 |
260 |
62 |
3.31 |
1.22 |
110 |
1.97 |
5.38 |
|
|
8.6 |
133 |
15 |
0.92 |
0.49 |
110 |
3.27 |
6.47 |
|
|
8 |
119 |
13 |
0.85 |
0.41 |
120 |
3.15 |
6.72 |
|
|
12.9 |
178 |
19 |
1.36 |
0.45 |
130 |
2.37 |
7.25 |
|
|
8.3 |
439 |
59 |
1.11 |
0.41 |
130 |
0.69 |
1.89 |
|
|
19.3 |
218 |
30 |
2.67 |
0 |
100 |
0.00 |
8.85 |
|
|
14 |
193 |
|
0 |
1 |
120 |
|
7.25 |
|
|
10.1 |
301 |
41 |
1.6 |
0 |
70 |
0.00 |
3.36 |
|
|
9.6 |
169 |
18 |
1.09 |
0.74 |
89 |
4.11 |
5.68 |
|
|
11.1 |
366 |
|
0 |
0.99 |
130 |
|
3.03 |
|
|
7.5 |
735 |
120 |
1.27 |
0.46 |
100 |
0.38 |
1.02 |
|
Luxemburg |
14 |
174 |
|
0 |
0 |
120 |
|
8.05 |
|
|
6.1 |
208 |
30 |
0.85 |
0.17 |
120 |
0.57 |
2.93 |
|
|
10.3 |
258 |
21 |
1.24 |
0 |
100 |
0.00 |
3.99 |
|
|
6.9 |
192 |
25 |
0.83 |
0 |
90 |
0.00 |
3.59 |
|
|
15.3 |
140 |
|
0 |
0 |
110 |
|
10.93 |
|
|
21 |
505 |
|
0 |
1.51 |
120 |
|
4.16 |
|
|
14.9 |
485 |
74 |
2.28 |
0 |
100 |
0.00 |
3.07 |
|
Slovak Rep. |
11.3 |
146 |
59 |
4.69 |
0 |
130 |
0.00 |
7.74 |
|
|
13.7 |
523 |
83 |
2.17 |
0.99 |
130 |
1.19 |
2.62 |
|
|
13.2 |
244 |
|
0 |
0 |
120 |
|
5.41 |
|
|
6 |
178 |
23 |
0.83 |
0.25 |
110 |
1.09 |
3.37 |
|
|
7.1 |
326 |
39 |
0.84 |
0.37 |
120 |
0.95 |
2.18 |
|
|
5.6 |
80 |
105 |
7.3 |
5.01 |
90 |
4.77 |
7.00 |
|
|
6.1 |
386 |
52 |
0.75 |
0.21 |
113 |
0.40 |
1.58 |
|
|
14.9 |
682 |
46 |
0.94 |
0.52 |
113 |
1.13 |
2.18 |
Nilsson (1977)
studied the impact of having different speed limits on different highways
within the same highway category in
Another study by Nilsson
(1990) analyzed the impact of a reduction in speed limits from 110 kph to 90
kph (68.35, 55.93 mph) on motorways in the summer of 1989 in
Cameron, Newstead
and Vulcan (1994) conducted a study in
In 2003, Cameron
performed a total cost benefit analysis of the impact of increasing or decreasing
speed limits on the overall economic costs. The author concluded that if the
speed limits were raised to 130 kph (80.78 mph) from the speed limit of 110 kph
for automobiles and 100 kph for trucks, the vehicle operating costs would
increase by 7.2% and the crash costs would increase by 89.4%. Whereas the time
savings, due to higher speed limits, would decrease the time cost for the
public by 16.9%. Overall, the total economic cost was estimated to increase by
2.2%, from $288.8 million to $295.25 million. It was also observed that having
a uniform speed limit of 110 kph for automobiles and trucks could reduce the
overall cost. However, the optimum speed differed substantially by vehicle type
and it was estimated that a speed limit of 120 kph (74.57 mph) for automobiles
and 95 kph (59.03 mph) for trucks would minimize the economic costs.
Fieldwick (1987) conducted
a global study to estimate the effect of speed limits on road casualties using
1984 accident data. The data collected from 20 European countries and the
Elvik and Vaa (2004) analyzed the results of many studies conducted worldwide to assess the impact of changes in speed limits on the number of accidents and on the average traffic speed. Based on a meta-analysis, it was concluded