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13 for RTM using a treatment-comparison experimental design "migrate" to other sites; thus, it is conceivable that sites adja- is achievable only if sites are randomly assigned to a treat- cent to PRPM installation sites will experience such migra- ment and comparison group--a desideratum that is difficult tion effects. to accomplish in road improvement programming. Alterna- tively, there will be no RTM concern if the fact that a site had a higher than usual crash history is not used in the selection 2.3 LITERATURE REVIEW of the sites for treatment. For example, in the "before and OF HUMAN FACTORS ISSUES AND PRPMS after design with yoked comparison" used by Griffin (15) and The following subsections review the human factors issues Orth-Rogers and Associates (18), RTM bias is thought to be related to the use of PRPMs: eliminated by not using the number of crashes as a criterion for selecting sites. It is unclear how RTM bias was eliminated. Driver needs with respect to delineation and visibility, This strategy, however, is not realistic because it defeats the Visibility of PRPMs, and purpose of safety improvement programs since measures are Driver behavior in response to PRPMs. likely to have the largest safety benefits where a safety con- cern is manifested in a high crash frequency. 2.3.1 Driver Needs with Respect to Delineation and Visibility 2.2.2.4 Other Measures Simultaneously Applied Pavement markings and delineation devices provide an Zador et al. (21) acknowledged the difficulty of identify- important guidance function for drivers, especially at night. ing the effect of one treatment when multiple treatments have Pavement markings and delineation devices provide drivers been applied. This difficulty presents the following method- with information about the vehicle position within the lane ological challenge: to discard data where changes in addition and information about which lanes are available for use. to PRPM installation occurred during the study period or to Pavement markings and delineation devices also provide the try to isolate the effects due to PRPMs. For the latter option, driver with a preview of upcoming changes in the roadway a promising methodology recently applied by Feber et al. (22) geometry, including curves, lane drops, narrowing, the start could be considered. and end of passing zones, crosswalks, and intersections. There is a perception-reaction time delay between seeing a 2.2.2.5 Selection of the Comparison Group-- change in the road path and responding to it and between the Problem of Spillover making a steering input and the vehicle responding; there- and Migration Effects fore, several seconds of preview are required for good lane positioning. Good delineation generally results in better Treatment-comparison experimental designs are commonly driver performance and greater driver comfort. used to control for effects not due to the treatment. The treat- Driver requirements for delineation have been established ment effects would be underestimated if, as some of these through studies of lane tracking given various driver preview studies have found, there were a decrease in target crashes at distances and through studies involving the recording of driver comparison sites that was due to spillover effects of the treat- eye movements. Driver preview distance may be modified ment. Measures such as red light cameras are believed to by blocking parts of the forward view through the windshield have such effects. or by simulating reduced visibility conditions in a driving The importance of this point is emphasized by Orth-Rogers simulator. In actual vehicles on a tangent section of road, and Associates (18) in their analysis of the Pennsylvania data. McLean and Hoffman (23) found that, at 31 mph (50 km/h), As indicated earlier, the authors suspected that PRPMs may improving sight distance beyond 2 seconds did not further have had a positive effect on the daytime crashes used for the improve lane position control. On a highway, at speeds of comparison group that generated the result that PRPMs 50 to 68 mph (80 to 110 km/h), eye movement recorders caused only a marginal decrease in nighttime crashes. The showed that drivers looked about 3 seconds ahead of the authors further concluded that if this impact on the compari- vehicle (24). According to a Commission Internationale de son group were true, then the fundamental basis of the analy- l'Eclairage (International Commission on Illumination, or sis conducted by Griffin (15) as well as on their own study is CIE) report on visual aspects of road markings (25), Farber questionable. et al. (26) found that a minimum of 5 seconds' preview time In contrast to the underestimation caused by spillover, treat- was necessary to allow for efficient, anticipatory steering ment effects would be overestimated if there were crash behavior. "migration" (i.e., an increase in crashes at the comparison sites Based on these and other studies, the CIE report (25) rec- due to the compensatory behavior of drivers). The installation ommends a minimum practical preview time of 3 seconds of all-way stop control and other speed-control measures are and a desirable preview time of 5 seconds. The sharper the believed to sometimes cause vehicles, and therefore crashes, to curve, the greater the preview distance required to allow for