Rural, two-way stop controlled intersections present a significant challenge for traffic safety. For the ten year period of 2002 to 2011 in Minnesota, 43 percent of all intersection crashes occurred at unsignalized intersections. However, for this same period, 65 percent of fatal and serious injury intersection crashes occurred at unsignalized intersections. Rural, two-way stop intersections accounted for 76 percent of these fatal and serious injury intersection crashes at unsignalized intersections. Right-angle type crashes accounted for the largest percentage, by crash type, of these fatal and serious injury crashes at rural, two-way stop intersections. The problem is clear. The focus of intersection safety needs to be addressing right-angle crashes at rural, two-way stop intersections (1). Research in Minnesota suggests that approximately 60 percent of right-angle crashes at rural intersections involved a driver that came to a stop and then pulled out in front of a vehicle on the major road whereas 26 percent of these crashes involved a driver running the STOP sign (2). This suggests the focus should be to assist drivers on the minor road in judging appropriate gaps.
An emerging technology to treat rural, two-way stop intersections are new rural intersection conflict warning systems that are based on Intelligent Transportation Systems (ITS). In Minnesota, the Local Road Research Board (LRRB) funded a research project to design and implement in the field a rural intersection conflict warning system. Named the ALERT System (Advanced LED WaRning System for rural InTersections), it aggressively adopts recent ITS technologies and utilizes solar powered renewable energy, LED integrated signs, non-intrusive vehicle detection, and wireless communications between the devices. The goal of the project was to improve intersection safety, utilize “off-the-shelf technology”, and create a low cost system that is easy to install, operate and maintain by local agencies.
The ALERT System was installed at a rural, two-way stop intersection near Duluth, Minnesota in 2012. The speed limit on both roads was 55 mph. The Annual Average Daily Traffic (AADT) was 970 vehicles per day for the major road and 570 vehicles per day for the minor road. The ALERT System layout, as shown in the diagram below, was composed of three dynamic flashing warning signs, two dynamic flashing STOP signs, and six detectors.
On the minor road, a vehicle was first detected at the stop ahead warning sign. This detection activated the flashing STOP sign for a fixed time period based upon the typical deceleration of a vehicle to a stop condition. Once the vehicle arrived at the STOP sign, another detector activated the flashing CROSS TRAFFIC warning sign for the entire time the vehicle was detected at the STOP sign. On the major road, a vehicle approaching the intersection was detected and activated the flashing VEHICLE APPROACHING warning signs for a fixed time period based upon the typical time for that vehicle on the major road to arrive and pass through the intersection.
Crash data was not used in the analysis because there were no reported crashes in the before period. Instead, the research analyzed four surrogate metrics. The first was vehicle speeds on the major road. Second was the wait time for vehicles on the minor road at the STOP sign. Third was the percentage of roll-throughs for vehicles on the stop-controlled minor road. And fourth, a mail-in survey of local residents. Video cameras were used to observe and measure driver behavior on the major road and at the intersection. Video data was collected for 42 days before the ALERT System was installed and 259 days after the installation.
ALERT System Layout Diagram
Analysis for speed, wait time and roll-throughs were all analyzed during conflict and non-conflict times. A conflict was defined as when a vehicle was stopped on the minor road at the STOP sign and a vehicle on the major road was approaching the intersection. A non-conflict was defined as the absence of either a vehicle stopped on the minor road at the STOP sign or a vehicle on the major road approaching the intersection, but not both.
The speed analysis was used as a surrogate measure of a driver’s recognition of a potential conflict at the intersection ahead due to a vehicle stopped on the minor road at the STOP sign. A decrease in these vehicle speeds is assumed to correlate to an improvement in conflict recognition whereby the driver attempted to increase the amount of time in which to react to perform a successful evasive maneuver if necessary. The average speed for vehicles on the major road approaching the intersection before the ALERT System was installed was 52.0 mph. After the ALERT System was installed, the average speed of these vehicles during non-conflict periods was 51.6 mph, but during conflict periods the average speed was 48.7 mph. This means the ALERT System was able to reduce the average speed of vehicles on the major road approaching the intersection by 3.3 mph (52.0 mph to 48.7 mph) during a conflict.
The wait time analysis was used to determine if there was a change in the amount of time that a driver was stopped on the minor road at the STOP sign when there was a vehicle on the major road approaching the intersection. This analysis was used as a surrogate measure of a driver’s recognition of an acceptable gap. A longer wait time during a conflict is assumed to correlate to an improvement in gap recognition. The average wait time before the ALERT System was installed was 2.0 seconds. After the installation, the average wait time was 2.5 seconds during non-conflict periods and 3.9 seconds during conflict periods. This means the ALERT System was able to increase the wait time for a driver stopped on the minor road at the STOP sign when there was a vehicle on the major road approaching the intersection by 1.9 seconds (2.0 seconds to 3.9 seconds).
The roll-through analysis considered all three turning movements of vehicles stopped on the minor road at the STOP sign and then proceeding into the intersection. A stop was defined by a vehicle that came to a complete stop and also a vehicle whose relative velocity was below an established threshold that was determined by the research team, referred to as a “rolling stop”. All other vehicles were defined as a roll-through. This analysis was used to assess whether the ALERT System had a negative effect on a driver’s compliance with the STOP sign during non-conflict periods and, conversely, if drivers had a better recognition of an unacceptable gap during conflict periods. If the percentage of roll-throughs increased during non-conflict periods, it is assumed to correlate to drivers using the ALERT System as a de-facto traffic signal. Conversely, a reduction in the percentage of roll-throughs during conflict periods is also assumed to correlate to an improvement in gap recognition. Before the ALERT System was installed, the percentage of roll-throughs for all vehicle movements from the minor road was 28 percent. The largest percent of roll-throughs occurred for right-turn movements and the least percent occurred for left-turn movements. After the ALERT System was installed, the percent of roll-throughs was 16 percent during non-conflict periods and one percent during conflict periods. Roll-throughs were nearly eliminated during conflict periods. The largest percent of roll-throughs during the non-conflict period occurred for right-turn movements and the least percent occurred for through movements (a flip from the before period). There effectively was no difference in the percent of roll-throughs during conflict periods by the type of turning movement.
And finally, a mail-in survey was sent to residents living within a two mile radius of the study intersection. A total of 206 surveys were sent out and a total of 119 were returned for a 58 percent response rate. The first question asked how frequently they drive through the intersection. The majority (47 percent) stated two times per day. The second question was broken into four statements and asked the respondent to assess how strongly they agreed or disagreed. See the following table for the results. For the final question, a cumulative response of 87 percent ranked the effectiveness of the system as “Excellent” (52 percent) or “Good” (35 percent).
In conclusion, the evidence suggests this system was able to effect a positive change in driver behavior for both the major road and minor road during a conflict. In the NCHRP Report 500, Volume 5, which lists safety strategies for unsignalized intersections, two of the safety strategies are assisting drivers on the minor road in judging appropriate gaps and reducing the operating speed of vehicles on the major road. It appears the ALERT System was able to accomplish both of these objectives. However, it should be cautioned that there are still questions and concerns surrounding the behavior of drivers on the minor road when there is no conflict with a vehicle on the major road. It appears drivers are blending the sign definitions of the Manual on Uniform Traffic Control Devices whereby the driver treats the STOP sign as a warning sign and the VEHICLE APPROACHING warning sign as a regulatory sign. In effect, during non-conflict periods, drivers appear to treat the system as a de-facto traffic signal. However, even in the worst case scenario during non-conflict periods, the roll-through percentage is lower than before the ALERT System was installed (16 percent versus 28 percent). It is recommended that future human factors research be performed on this observed phenomenon.
- Husam Ismail, Research Assistant, University of Minnesota Duluth
For further reading, the full research report can be accessed from the Minnesota LRRB website at www.lrrb.org. The report is expected to be published on this website by spring 2014. A video summarizing this research project is available for viewing at mntransportationresearch.org/.
1. 2011 Minnesota Intersection Green Sheets. http://www.dot.state.mn.us/stateaid/trafficsafety.html. Accessed on January 21, 2014.
2. Harder, K. Bloomfield, J. and Chihak, B. 2003. Reducing Crashes at Controlled Rural Intersections. Minnesota Department of Transportation Report Number 2003-15.