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Roadway Design: System-Wide Safety Treatments and Design Guidance for J-Turns
This report seeks to document ways to reduce crashes and fatalities from wrong-way crashes, horizontal curves, intersections, and J-turns.
Report No.: MoDOT cmr 16-013
The Missouri Department of Transportation (MoDOT) sponsored this research project to investigate treatments that can reduce crashes and fatalities to further the goal of the Toward Zero Deaths (TZD) initiative. One major objective was to synthesize the literature and state of the practice related to system-wide safety treatments and document the treatments’ effectiveness. Specifically, the objective was to examine those treatments that have not already been implemented in Missouri. Another major objective was to provide guidance on the design of the J-turn intersection, which eliminates or reduces crossing conflicts.
A synthesis of system-wide safety treatments from other states and countries was conducted. The safety effectiveness, implementation guidelines, limitations, costs, and concerns of the treatments were documented. The identified safety treatments are consistent with the “Necessary Nine” strategies identified in the Missouri Blueprint. Accordingly, the synthesis covered three areas: (1) horizontal curves, (2) intersections, and (3) wrong-way crashes. The reviewed treatments include signing, geometric design and access management, intelligent transportation systems (ITS), pavement markings, and signal control enhancements to improve safety. This synthesis provides a systematic method for selecting system-wide treatments for future deployments in the state of Missouri.
Countermeasures related to signage, design, ITS, and drivers were reviewed to address wrong-way crashes. Innovative signage strategies including lowering the height of signs, deploying oversized signs, providing illumination, and doubling the number of signs are low-cost solutions that can be deployed across the system. Design countermeasures such as avoiding left-side exit ramps, using raised medians on crossroads, and improving sight distance are also recommended. ITS technology options, due to their higher costs, may not be suitable for system-wide deployment but are appropriate for isolated treatments. Detection and alert systems based on video radar or in-pavement sensors have been piloted in a few states.
Countermeasures targeting horizontal curve crashes may include signage treatments that exceed the minimum signage and device requirements recommended by the Manual on Uniform Traffic Control Devices (MUTCD) for horizontal curves. Such treatments include improved curve signing through the use of additional chevrons, flashing beacons at sharp curves, dynamic curve guidance systems, and dynamic speed warning systems. Other recommended horizontal curve safety treatments include pavement treatments such as speed reduction markings, warning symbols painted on the pavement, and high-friction pavement treatments. MoDOT has successfully utilized two pavement marking treatments in the past: wider edge lines and rumble strips/stripes.
Treatments to enhance signalized intersection safety include increasing clearance intervals, changing left turns from permissive to protected-permissive, installing flashing yellow arrows, providing dynamic signal warning, installing red light cameras, and improving signal visibility. Based on the safety effectiveness reported in literature, providing dynamic signal warning and improving signal visibility are recommended for future consideration as system-wide treatments at signalized intersections in Missouri. At stop-controlled intersections, the use of bigger signs, LEDs, and flashing beacons was found to reduce crashes due to the increased visibility and illumination of signs.
In the last few years, MoDOT has replaced several high-crash intersections on rural highways in the state with J-turns. Given their safety effectiveness and low cost, J-turns have become a preferred alternative to grade-separated interchanges for replacing high-crash two-way stop-controlled intersections on high-speed highways. Unfortunately, national guidance on the design of J-turns is very limited. For example, there are no recommendations on the spacing between the main intersection and the U-turn. Similarly, there is no guidance on when acceleration lanes are recommended, i.e., at what level of traffic volume. This project addressed this gap in practice by developing guidance on spacing and acceleration lanes. A thorough examination of crashes that occurred at 12 existing J-turn sites in Missouri was conducted. The objective of this review was to determine if the crash frequencies and types of crashes were influenced by the aforementioned design parameters.
The crash review revealed the proportions of five crash types occurring at J-turn sites: (1) major road sideswipe (31.6%), (2) major road rear-end (28.1%), (3) minor road rear-end (15.8%), (4) loss of control (14%), and (5) merging from U-turn (10.5%). Vehicles merging with the major road traffic or changing lanes to access the U-turn lane caused most of the major road sideswipe and rear-end crashes. Other common contributing factors included driver inattention and the large speed difference between the merging vehicles from the minor road and the vehicles on the major road. Crash rates, expressed as per million vehicle miles of travel, decreased with an increase in the U-turn spacing for both sideswipe and rear-end crashes. A longer spacing allowed merging vehicles to reach major road operating speeds, thus making it safer to follow other vehicles in the lane and to make lane changes. J-turn sites with a spacing of 1,500 ft or greater experienced the lowest crash rates.
In addition, traffic simulation experiments were conducted to study the effect of different design parameters and traffic volumes on the safety of the J-turn design. A base simulation model was created and calibrated using field data collected during a previous MoDOT project on J-turns. The calibrated model was then used to study various combinations of major road and minor road volumes and design variables. For all of the studied scenarios, the presence of an acceleration lane resulted in significantly fewer conflicts. Therefore, acceleration lanes are recommended for all J-turn designs, including those at lower volume sites. Second, while a spacing between 1,000 and 2,000 ft was found to be sufficient for low-volume combinations, a spacing of 2000 ft is recommended for medium- to high-volume conditions.