An Application for Driving Simulator Technology: An Evaluation of Traffic SignalDisplays for Protected-Permissive Left-Turn Control

The objective of this paper was to determine the safety and effectiveness of selected protected/permissive left-turn (PPLT) signal displays through driver comprehension evaluations. Driver evaluations were conducted using full-scale, fixed-based, fully-interactive driving simulators located at the University of Massachusetts and the Texas Transportation Institute. PPLT displays were also evaluated in a static environment to provide comparison data. In an effort to evaluate driver’s comprehension of the various PPLT signal displays in use, several traditional study methods have been employed. Commonly, pen and paper comprehension tests are used in which the driver simply marks what he/she believes to be the correct answer. The critique of this methodology has focused on the belief that the study responses provided might not be consistent with the decision made in the actual driving environment. To add more realism to the experiment, computer technology has been employed by providing static photos of actual driving environments and superimposing PPLT signal displays within. Although this technology is believed to be a major step forward in experimentation, the static nature and lack of dynamic cues may still lead drivers through a different decision process. This paper describes the use of a full-scale dynamic driving simulator as a tool for evaluating driver comprehension of PPLT signal displays. Drivers traversed through a series of signalized intersections created as part of a comprehensive visual world. Road signs were provided in the simulation to direct the driver through a series of left, right, and thru movement maneuvers. Traffic signals varied at each intersection providing the test variable. Vehicles were always present in the opposing lanes at signalized intersections. Driver comprehension was determined from the distribution of correct and incorrect responses to the different signal scenarios presented. Findings from the driving simulator study showed a high level of comprehension, consistent with the responses one would expect in an actual roadway environment. Drivers were found to use many cues, not always including just the left-turn signal display itself, to make turn decisions. A direct comparison of responses in the simulator and static evaluations was completed by cross-analyzing individual driver’s responses in each methodology. The query was undertaken to determine if drivers’ comprehension from the static evaluation was consistent with each driver’s action in the dynamic simulation. Of the 353 fail critical error responses in the static evaluation (errors that may have lead to a crash), 279 of these did not take place at identical scenarios in the simulator environment. Only 19 percent of the 353 pairs resulted in fail-critical responses in both the simulator and static evaluation. Overall, the results obtained from this experiment indicate that the driving simulator is an effective method of evaluating driver comprehension of traffic signal displays. Specifically, the results from the driving simulator much more closely mirror behavior in the real world than do the results from the static evaluation. Furthermore, these results may help explain why the low level of comprehension reported in previous research, which used static evaluations to assess driver comprehension of PPLT signal displays, is not consistent with left-turn crash frequencies. Specifically, what drivers say they will do and what they actually do in the driving environment are not always consistent.