United States. Department of Transportation. Federal Highway Administration
Abstract
DTFH61-90-C-00038Four on-road experiments investigated whether the assumed values for driver perception-reaction time (PRT) used in AASHTO design equations adequately represent the range of actual PRT for older drivers. The Case III (stop controlled) intersection sight distance (ISD) experiment found that older drivers did not have longer PRT than younger drivers; 85th percentile PRT closely matched the AASHTO design equation value of 2.0 s. In the stopping sight distance (SSD) experiment involving brake reaction times to an unanticipated event (crash barrel suddenly rolling toward roadway), there were apparent differences in the distribution of PRT among age groups. Younger drivers accounted for most of the fastest PRT, but there were no age differences in the 50th or 85th percentiles. All observed PRT were encompassed by the current AASHTO design value of 2.5 s. The decision sight distance (DSD) experiment measured when drivers recognized the need to make a lane change maneuver, from the first visibility of the roadway cue used by the driver. Although observed DSD values were generally longer with increasing driver age, the 85th percentile PRT for all age groups were well below AASHTO design assumptions. The final experiment collected judgments about the acceptability of gaps and lags in traffic. Younger subjects accepted shorter gaps and rejected lags later than older subjects. Based on these findings, and consideration of the implications of changes in PRT for sight distance requirements, no changes to design PRT values, based on older driver performance, were recommended for ISD, SSD, or DSD. Alternative models for ISD, based on gap acceptance or lag rejection, were explored. Based on limited data, it is not clear whether these models offer any significant benefits to the current AASHTO model
