Researchers Studied How Drivers Reacted to Speed Warnings on Curved Highway Interchange Ramps in Michigan.
Michigan, United States
Evaluation of Dynamic Speed Feedback Signs on Freeway Interchange Ramps
Summary Information
Lane departure crashes at freeway ramps is a common safety issue and dynamic speed feedback sign (DSFS) is a promising countermeasure to reduce curve speeds on freeway interchange ramps. DSFS uses real-time speed detection to provide targeted warning messages to drivers when they are moving above the posted ramp advisory speed limit. However, their use on freeway interchange ramps has been limited.
This study evaluated the performance of a DSFS system on six interstate and state highway interchange ramps in Michigan on various measures of driver behavior, particularly speed on the approach and entry to the ramp curve. The field tests were conducted over a 14-month period, and data were collected first without the DSFS present and then after the installation of the DSFS or after modifying the DSFS setup or operation. Three commercially available speed feedback signs, which varied in size, border type, and radar detection range, were utilized during the field evaluations. The study assessed various aspects related to the design, operation, and/or installation of the DSFS, which included sign messaging strategy, longitudinal positioning of the sign relative to the ramp curve, lateral positioning of the sign with respect to the side of the ramp, sign dimensions and other physical characteristics, radar activation range, time of day, interchange type, and temporal changes in driver behavior.
- Ensure a minimum of 250 feet radar activation range. This study found strong sign-related effects associated with the radar detection range. With the DSFS installed near the start of the curve, the lowest curve entry speeds were observed for cases where the DSFS was positioned so that feedback message was activated for vehicles within 250 to 400 feet of the start of the curve.
- Place the DSFS within 250 feet after the start of the ramp to slow down drivers. When signs were placed within 250 feet after the beginning of the freeway ramp, drivers slowed down by 1.5 to 4.0 mi/h (miles per hour) in comparison to when the signs were not present. In contrast, when the signs were placed at greater distances upstream of the curve, there was no significant effect on speeds.
- Install the DSFS on the right-side of the ramp. This study found that both the right side-mounted and forward-mounted (i.e., ramp gore area) installations resulted in similar curve entry speeds. Installing the sign in the traditional right-side mounting position is recommended as installing in the goal area can increase likelihood that an errant vehicle may collide with the sign in this position. However, the DSFS may be installed within the gore area of the ramp between the exit sign and the initial chevron if the traditional right-side mount is restricted due to obstructions or terrain issues.
- Choose DSFS that includes a full matrix amber LED feedback display capable of displaying characters that are a minimum of 15 inches in height. The findings of this study showed that sign size and color do not have large effects on speed reductions in general, yet yellow signs are recommended as they may increase visibility in cluttered areas. Both 15- and 18-inch signs warranted the same driver response and thus may be used interchangeably.
