1 Introduction
1.1 General
An intersection in highway engineering may be defined in many ways. The American Association of State Highway and Transportation Officials (AASHTO) refer to an intersection by its physical and functional areas [AASHTO 2001]. The physical area of the intersection is the area where two or more highways join or cross including the roadway and roadside facilities for traffic movements. The functional area of the intersection includes the areas of any auxiliary lanes and their associated channelization (see Figure 1.1). The Highway Capacity Manual (HCM) implies that highway intersections are subsets of interrupted-flow facilities that have controlled and uncontrolled access points that can interrupt the traffic flow [HCM 2000]. These access points include intersections without any control, traffic signals controls, stop signs controls, yield signs controls, modern roundabouts or other controlling devices that predominantly stops/slows traffic irrespective of the amount of traffic. Others define intersections as an area of concentrated conflicts points between vehicles and, between vehicles and pedestrian [Kuciemba and Cirillio 1992].
In general, a roadway intersection can be said to be a junction where three or more roads/legs meet and provide an option for motorists to change directions. The result of the traffic flow through the intersection can otherwise be called the capacity of the intersection, i.e., “the maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a point or uniform section of a lane or roadway under the prevailing roadway, traffic and control conditions” [HCM 2000]. The maximum hourly rate depends on the interactions between the vehicles at the intersection, geometry, intersection control and the environmental characteristics of the roadway [AASHTO 2001; HCM 2000].
The main objective of a traffic control device (TCD) at an intersection is to provide appropriate regulatory, warning and guidance information to the motorists approaching or using the intersection, such that there is little or no violation in the expectancy of the drivers [TCD Handbook 1983].
Figure 1.1 Intersection physical and functional areas
(taken from AASHTO 2001)
Therefore, for
proper operation of the intersection relating to safety, cost and efficiency,
an appropriate and well-designed TCD should be properly installed at
appropriate intersections. The conventional types of intersection control are
yield control, two-way stop control (TWSC), all-way stop control (AWSC) and
traffic signal control (TSC). Apart from
these conventional types of TCDs, a unique type of intersection control -“the modern roundabout”- is being considered
an alternative for intersection traffic control in the
Delay is
considered to be a critical measure for evaluating the performance of the
intersection [HCM 2000; Wallace et al., 1998]. Many studies comparing the
viability of various alternatives for intersection control have been made since
the introduction of modern roundabouts in the
A study that compared the performance of various conventional intersection types with modern roundabouts using Signalized and Unsignalized Intersection Design and Research Aid (SIDRA) concluded that modern roundabouts are a better form of intersection control than other intersection types for many conditions [Sisiopiku and Heung-Un Oh 2001]. The conditions in the comparison included variations in volume levels, turning volume splits, number of approach lanes and lane width. The signalized intersection included a simple two-phase design; permitted left turns; right turns on red; and shared turn lanes.
This study concluded: [Sisiopiku and Heung-Un Oh 2001]
· AWSC resulted in greater delay under all conditions than compared with other intersection types.
· Yield and TWSC performed better than other intersection types under light traffic demand.
· Roundabouts and TSC alternatives with single lane approaches and heavy traffic volumes are viable alternatives when the flare effect is maximized.
· For intersections with two-lane approaches and heavy traffic volumes roundabouts show a better performance than other intersection types.
· For three lane approaches, especially with heavy traffic volumes, roundabouts are no longer good alternatives and traffic signals serve better than other types.
· For heavy left turns demands, roundabouts show superior performance than other types of intersection control in terms of delay and capacity.
· Roundabouts with two or three-lane approaches provide increased capacity.
The Florida Roundabout Design Guide [FDOT 1996] also compared roundabouts to signalized intersections and found that the performance of signalized intersections is superior under higher entering volumes in terms of delay; however, Akcelik (1997) reported that the FDOT study failed to consider the flare effects correctly [cited in Sisiopiku and Heung-Un Oh 2001].
An Informational Roundabout Guide implies that roundabouts may offer a better performance and capacity at places were minor road traffic is more than 10% of the entering volume [FHWA Guide 2000]. Also, the guide states that roundabouts are favorable alternatives for TWSC at intersections with significant left turners. When compared with an AWSC, the modern roundabout always performs better for any given traffic condition [FHWA Guide 2000]. When the major street approaches dominate, roundabout delay is usually lower than the delay incurred by the TSC operation particularly where there is a high proportion of left turns [FHWA Guide 2000]. More studies concerning this research are cited in the Chapter II of this report.
This research study adds more evidence that modern roundabouts are capable of performing better and safer, handle higher traffic loading more efficiently and have a higher design life (based on degree of saturation) than other conventional types of intersection controls. Also, modern roundabouts can be potential candidates for intersections that have wide medians.
1.2 Study objective
The objective of this study is to further develop a methodology developed by Kansas State University (KSU) researchers [Russell, Rys and Luttrell 2000] for analyzing the performance of a modern roundabout. Also, to compare a modern roundabout with other conventional intersection traffic control types at a high-crash, problematic intersection with a wide raised median, in a city where opposition to modern roundabouts was very strong before it was built. This research report covers a comparison of the operational and the safety performance at the study intersection (23rd Avenue and Severance street, Hutchinson, Kansas) for the before (TWSC) and after (modern roundabout) conditions. Also, other conventional intersection traffic control types (excluding the yield sign control) were assumed and results simulated and compared with the performance of the modern roundabout that was constructed at this site.
The fact that
this was the first modern roundabout in
1.3 Organization of the report
Chapter 1: Introduction
Chapter 2: Literature Review: This chapter covers the summary of the literature review done for conducting this research study.
Chapter 3: Experimental Design: This chapter covers a comprehensive summary of the study methodology adopted in this study.
Chapter 4: Data Collection: This chapter describes the study site and the summary of the data collected for this study.
Chapter 5: Data Analysis I – Operational Analysis: This chapter covers the summary of the operational analysis results obtained, both before and after the construction of the modern roundabout. The before condition was controlled by TWSC.
Chapter 6: Data Analysis II – Safety Analysis: This chapter covers the safety evaluation done for the study site both before and after the modern roundabout was built.
Chapter 7: Data Analysis III - Theoretical Analysis: This chapter presents the summary of the results for the theoretical analysis conducted to compare various types of intersection traffic control (TWSC, AWSC and TSC) including the modern roundabout operating under state fair traffic. Also, comparison of operational performance under increased traffic loading and a design life analysis for these intersection traffic control types are presented.
Chapter 8: Summary, Conclusions and Areas of additional research needs are presented in this chapter.
1.4 Units of measure
The units of measure to convert
from foot-pound-second (
meters = 0.30 * feet..………………………………….………………………..(1.1)
km/h = 1.61 * mph
………………………………………………..…………...(1.2)
2 Literature Review
2.1 General
The primary objective of this study is to compare the performance of the modern roundabout with the previous intersection control device (TWSC), installed at the study intersection based on operational and safety benefits. The secondary objective is to assume other conventional intersection types and determine which among them would be an appropriate traffic control at the study intersection based on the operational benefits. This chapter summarizes the literature review done for this study.
2.2 Intersection design characteristics
It is understandable that any
traffic related phenomena is a direct or indirect result of the interaction
between ‘the driver’, ‘the roadway’, ‘the vehicle’ and ‘the environment’. Every
year it is estimated that more than 40,000 people are being killed in the
Design Vehicle: A selected representative class of vehicles of certain weight, dimensions and operating characteristics such as turning radii, vehicle performance, acceleration, deceleration, etc., that are used for establishing highway design controls for accommodating them on the highway is known as a design vehicle. There are four general classes of design vehicles:
1. passenger cars: This includes cars of all sizes, sport/utility vehicles, minivans, vans and pick-up trucks.
2. buses: Inter-city motor coaches, city transit, school and articulated buses
3. trucks: This include single-unit truck, truck tractor semi-trailer and full trailers, and
4. recreational vehicles: This includes motor home, cars with camper trailers, boat trailers etc.
Each of the above mentioned classes of vehicles have different qualities. In the design of any highway facility, the AASHTO design guide (2001) recommends using the largest design vehicle likely to use the intersection. The knowledge of design vehicle is important when the curb radii are designed for an intersection.
Design Driver: Driver error is one of the prime reasons for auto crashes [Pline 1992]. Human factors relating to highways is concerned with the capabilities and limitations of the road user. A great many differences exist in humans; for example, information handling for older drivers may take more time than younger drivers for the same situation on a highway. An intersection design is said to be compatible for the driver if he/she feels safe and comfortable in using the intersection, otherwise, it is not compatible.
“In traffic engineering, 85th percentile is often used as a cut-off for determining things such as speed limits, sight distance etc. There is no such person as “average driver” or 85th percentile driver, as all individuals will vary in driving abilities”. Therefore, sufficient care should be taken to make the design is suitable for all users [Pline 1992].
Design speed: Speed reduces the visual field, restricts peripheral vision, and limits the time available for drivers to receive and process information. Speed at which drivers can easily process information within a cone of clear vision is known as the design speed. This is usually considered in designing the approaches of intersections, curves and placements of signboards.
Sight
distance: Sight distance is provided continuously along each highway or
street so that drivers have a view of the roadway ahead that is sufficient for
them to make safe driving decisions and take proper actions. AASHTO design
guide (2001) recommends that each quadrant of an intersection should contain a
triangular area free of obstruction that might block an approaching driver’s
view of potential conflicting vehicles.
According to AASHTO design guide (2001), the intersection design should consider the following five basic elements:
1. Human factors: This includes driving habits, ability of drivers to make decisions, drivers expectancy, decision and reaction time, conformance to natural paths of movement, pedestrian use and habits and bicycle traffic use and habits
2. Traffic considerations: This includes design and actual capacities, design-hour tuning movements, size and operating characteristics of vehicle, variety of movements (weaving, diverging, merging and crossing related to driving habits), vehicle speeds, transit involvement, crash experience and bicycle & pedestrian movements
3. Physical elements: This features character and use of abutting property, vertical alignments at the intersection, sight distance, angle of the intersection, conflict area, speed-change lane, geometric design features, traffic control device, lighting equipment, safety features, bicycle & pedestrian traffic, environmental factors and cross walks.
4. Economic factors: This includes cost of improvements, and effects of controlling or limiting right-of-way on abutting residential or commercial properties where channelization restricts or prohibits vehicular movements
5. Functional intersection area: Illustrated previously in Figure 1.1
2.3 Types of intersection controls
Intuitively, the most economical method to control the intersection is to allow the drivers themselves to control all movements. In the beginning of automobile travel this was acceptable. However, an increase in the number of road users led to chaos and conflicts that ultimately ended in crashes leading to property damages/injuries/loss of life. In order to enhance safety and operational efficiency, stop controls were introduced. This promoted an orderly flow and also increased operational efficiency more than at the uncontrolled intersection. TWSCs were introduced at intersections where there were low volumes on the minor roads. AWSCs were used at intersection if the TWSC caused excessive delays for the users on the minor roads or if the volumes on all the approaches were about equal. Also, AWSC was introduced to reduce right angle collisions and severity of crashes [Brigla Jr, 1982]. TSCs were provided if there were excessive traffic volumes such that AWSC would create significant delays to all the approaches. Yield signs were also placed as a control at intersections where there were lower volumes.
After many studies and experience focusing on the operational improvement of the intersection and the safety of the road users, the Manual for Uniform Traffic Control Device (MUTCD) specifies warrants such that a specific traffic control device at intersections could be installed. Stop signs come under the category of regulatory signs that inform traffic laws or regulations and indicate the applicability of legal requirements that would not otherwise be explicit to the highway users. The warrants for a stop sign is intended for application at places where traffic movement is unduly hazardous, where roads enter a through highway or street, where a signalized intersection is unsignalized and where serious crash records indicate need for regulatory signs [MUTCD 2000]. The current MUTCD (2000) provides eight warrants or thresholds to justify a traffic signal at a particular location. The following section highlights the purpose or the intentions for a specific warrant and cites some thresholds for specific conditions from the manual [MUTCD 2000]. The readers are requested to refer to the MUTCD 2000 for comprehensive and specific considerations.
Warrant 1: 8-hour vehicular volume: This is intended for application where a heavy traffic volume at the intersection is a primary reason for traffic signal control (condition A) or where traffic volume on the major street is so heavy that the minor street experiences excessive delay (condition B). For example: Condition A - The minimum vehicular volume during an eight hour period for an intersection with single lane approaches must exceed 500 vehicles per hour on the major street and 150 vehicles on the higher-volume side street. Condition B – The total vehicles per hour on the major streets (both approaches) during an eight-hour period must exceed 750 and exceed 75 vehicles per hour on the higher-volume minor approach.
Warrant 2: 4-hour vehicular volume: This is also intended for application where heavy traffic volume at intersection is a primary reason for traffic signal control. Here it takes into account the four-hour traffic volume on an average day. It requires a minimum of 80 vehicles per hour for the minor road with a single lane
Warrant 3: Peak-hour vehicular volume: This warrant accounts for intersections where the minor road faces more delays when entering or crossing the major street during a one hour period on an average day. It requires a minimum of four vehicle-hours of delay for a one lane approach on the minor approach controlled by stop signs, a minimum of 100 vehicles per hour on the delayed approach (>4 vehicle-hours) or 150 vehicles per hour for two moving lanes and a minimum of 800 vehicles serviced during that hour for intersections with four or more approaches.
Warrant 4: Pedestrian volume: This is intended for intersections where the pedestrians experience more delay due to heavy traffic volume on the major street. It requires 100 pedestrians during any four hours or 190 pedestrians or more during one-hour. Also a minimum of 60 gaps per hour of adequate length in the traffic stream to allow pedestrian to cross, during the same period for the pedestrian volume criterion mentioned above must be satisfied.
Warrant 5: School crossing: This is to determine if a traffic signal needs to be installed near schools. It requires a minimum of 20 students during the highest crossing hour and also the frequency and the adequacy of gaps in the vehicular traffic stream is less than the number of minutes in the same period.
Warrant 6: Coordinated Signal System: This is intended to address progressive movement of traffic (i.e. proper platooning of vehicles). Coordination between the signals shall be provided if the signalized intersection is less than 300 m (1000 feet)
Warrant 7: Crash experience: This warrant is intended for intersections where severity and frequency of crashes are the prime reasons for the implementation of a traffic signal. It requires a minimum of 5 crashes that are susceptible to correction by a traffic signal within one year.
Warrant 8: Roadway network: This is intended at intersections where the need for concentration and organization of traffic flow exists.
Traffic volume, as discussed in the earlier part of this section, constitutes a prime criterion in the choice of a particular traffic control device. Figure 2.1 shows the type of intersection control required for an observed peak hour volume on the major and minor roads of an intersection [HCM 2000]. It is to be noted that modern roundabouts can also be appropriate within those ranges.
Figure 2.1 Intersection traffic control types and
peak-hour volumes
Though the
abovementioned, conventional intersection controls may increase the operational
efficiency under certain, specific conditions, all are less effective than a
modern roundabout when safety issues are considered [Sisiopiku and Heung-Un Oh
2001; UTM 1996]. Flannery and Datta (1996) made an effort to determine the
effectiveness of modern roundabouts as a treatment for intersecting roadways.
Their study concluded that the reduction in crashes for roundabout sites was
between 60 and 70 percent and a modern roundabout would be an appropriate
intersection safety treatment [Flannery and Datta 1996]. Recent research [IIHS
July, 2001] reports that modern roundabouts perform better than other
conventional intersection traffic control types by reducing the delay and
proportion of vehicles stopped under a range of traffic conditions in the
2.4 Modern roundabouts
Most of the contents in this section are paraphrased from Modern Roundabout Practice in the United States, National Cooperative Highway Research Program Synthesis of Highway Practice (NCHRP) No. 264, [Jacquemart 1998] unless otherwise stated.
The idea of a one way rotary system was first
proposed in the
Figure 2.2 View
of
In
1929, Eno recognized that traffic in a traffic circle locks up at higher
volumes and pointed out that the main drawback could be due to the
yield-to-right rule (i.e. vehicles in the traffic circle yielded to the entering
traffic) and recommended a yield-to-left rule (i.e. entering vehicles yielded
to the circulating traffic). Instead, as an attempted solution to the locking
problem, larger rotaries were built with longer storage distances between
successive entries, but retaining the right-of-way rule. The locking problem
remained as the traffic volume increased. A negative effect was that these
larger circles led to (perhaps even encouraged) high-speed entering vehicles,
higher speeds on the circulating roadways, high-speed weaving maneuvers and
increased crash risk. Reluctant to reverse the right-of-way rule, and unable to
solve operational and locking problems, the
However, the United Kingdom (UK) continued research on traffic circles and came to a consensus, that a ‘give way’ rule for circulating traffic, otherwise called the offside priority rule in the UK, would make traffic circles operate more efficiently. Further research done by the Transport Research Laboratory (TRL) in the UK proved that the offside priority rule (entering vehicles yield to the circulating vehicles) would eliminate locking problems, increase capacity, reduce delay and also increase safety [Todd 1988, cited in Jacquemart 1998].
Studies and experiences in various developed countries, dating back to the beginning of the 20th century, have reshaped the concept of the older traffic circles, rotaries and gyratories into a more refined form of intersection control, known today as a “Modern Roundabout”.
2.4.1 Characteristics of modern roundabouts
(i)
Roundabout in
(iii)
Big traffic circle or rotary (Arc De Triomphe) in
Figure 2.3 Roundabout,
A modern roundabout is a type of intersection control that has a central island (can be circular or elliptical) and restricts the flow of traffic in a counter-clockwise direction (in countries that drive on the right side of the roadway) with entering vehicles yielding at entry to the circulating vehicles. Figure 2.4 shows the geometric elements that constitute a modern roundabout.
Figure 2.4 Geometric elements of a modern roundabout
(Source: FHWA Guide 2000)
· Circulating road width: The width of the circulating roadway on which the vehicles circulate to reach their preferred exits. It is the width between the outer edge of the roadway and the central island excluding the width of the truck apron.
· Inscribed diameter: The diameter measured between the outer edges of the roadway. This includes, the circulating roadway, truck apron and the central island.
· Entry and exit width: The perpendicular length from the right edge of the entry/exit to the intersection point of the left edge line and the inscribed line.
· Entry and exit radii: The minimum radius of curvature of the outside curb.
· Approach and departure width: The width of the approach/departure lane used by traffic stream to enter/exit the intersection.
This intersection control has superior operational characteristics (i.e. capacity, delay, queue length, proportion stopped, etc.,) and the capability of reducing the frequency of crashes and crash severity makes it safer than other TCDs [FHWA 2000; IIHS 2000; Russell, et al., 2000; Jacquemart 1998; Garder, 1998; Flannery, 2001; Austroads 1993; Garder et al., 2000; Flannery & Datta 1996; Alcelik and Besley 1998; HWS consultant group 2001].
An Insurance Institute of Highway Safety (IIHS) study have shown that total crashes decreased by 39%, injury crashes decreased by 76% and collisions involving loss of life were estimated to reduce 90% [IIHS 2000]. A study done by the Maryland Highway Administration at five of its modern roundabouts revealed a decrease of 63% in total crashes and a decrease of 86% in injury related crashes [Urban Transportation Monitor (UTM) 1999].
A research study conducted by the researchers at KSU for IIHS, studied the before and after performances at three intersections, one each in Kansas, Maryland, and Nevada, which were controlled by two-way stop signs before being converted to modern roundabouts [IIHS 2001]. The study concluded that the modern roundabouts at these three locations perform better than the two-way stop controls (TWSC) they replaced by reducing the average intersection delay by about 20% in each case and reducing the proportion of vehicles having to stop by 14% to 37% at the three sites [IIHS 2001].
Another study done by KSU researchers studied comparable intersection traffic control types (TWSC and AWSC) and found that a modern roundabout in Manhattan, KS performed equal or better than TWSC and AWSC intersection traffic control at low-volume intersections, considering safety and operation [Russell et al., 2000].
Delays
at modern roundabouts in the
Modern
roundabouts have been a great success in the