Evaluation of Dynamic Speed Feedback Signs on Freeway Interchange Ramps

Dynamic speed feedback signs (DSFS) are promising countermeasure to reduce curve speeds and subsequent lane departures at freeway interchange ramps, although their use in such contexts has been limited. Consequently, the impact of DSFS on driver performance at interchange ramps remains unproven. To that end, research was performed to determine the effect of DSFS installed at freeway interchange ramps on measures of driver behavior, particularly speeds approaching and entering the ramp curve. To accomplish this objective, a series of field evaluations were conducted at six freeway interchange ramps possessing significant horizontal curvature. Various DSFS configurations were tested during these evaluations, including: sign messaging strategy, longitudinal positioning of the sign with respect to the ramp curve entry point, lateral positioning of the sign with respect to the side of the ramp, sign dimensions and other physical characteristics, radar activation range, and temporal changes in driver behavior. Overall, the presence of a DSFS positioned near the start of the curve resulted in curve entry speeds that were, on average, 1.5 mph to 4.0 mph lower than without a DSFS present at the site. When the DSFS was present near the start of the curve, the lowest curve entry speeds were observed for cases where the feedback message activated when vehicles were within 250 to 400 ft of the start of the curve. Regarding DSFS lateral position, both the rightside-mounted and forward-mounted (i.e., ramp gore area) installations resulted in similar curve entry speeds. Furthermore, there were no discernable differences in curve entry speeds between 15-inch and 18-inch display panels, nor were speeds impacted by the inclusion of an advisory speed panel. In terms of feedback message, the most effective strategy was to display the measured speed alternating with a SLOW DOWN message. The effects on driver behavior associated with the DSFS were consistent between system interchanges and service interchanges, and across all vehicle types. Finally, there was no evidence of temporal changes in driver behavior during the initial 14 months of operation of the permanent DSFS installation evaluated here. Based on these findings, the continued use of DSFS as a speed reduction treatment at freeway interchange ramps is recommended. Additional guidance towards utilization of DSFS is provided within the report.