State DOT Researchers Tested Multiple DSFS Deployment Scenarios at Six Freeway Interchange Ramps in Michigan.
Saugatuck
Evaluation of Dynamic Speed Feedback Signs on Freeway Interchange Ramps
Summary Information
Dynamic Speed Feedback Signs (DSFS) can be used as countermeasures for lane departure crashes at freeways that use real-time speed detection to provide targeted warning messages to drivers. These signs are typically implemented in work zones, school zones, and general municipal speed control areas.
This study aimed to identify effective uses for DSFS when used as a speed control countermeasure on freeway ramps with sharp horizontal curvature. Field evaluations were performed in Michigan to examine the impacts of message type, longitudinal position, interchange type, time of day, sign size, radar detection range, lateral sign position, temporal changes, and the impact on mainline traffic flow.
METHODOLOGY
This study examined three commercially available DSFS signs at six different freeway interchange ramps in Michigan. The evaluation tested several aspects of the DSFS, including physical characteristics (display size and border type), lateral installation position (side-mount vs. forward-mount), and vehicle detection range of the radar. Data were collected at each study location in two phases: (i) First phase was under the existing site conditions without the DSFS present, and (ii) Second phase was after DSFS installation or after modifying the DSFS setup or operation. Following the first phase, the DSFS were installed at each site by the Michigan Department of Transportation crews, while the existing signage at each site was not modified in any way. Data for the second phase were collected seven days after the sign installation to allow for drivers to become accustomed to the device. Three different techniques were used to collect vehicular speed data collection: (i) cameras, (ii) handheld LiDAR, and (iii) speed trailers. The selection of the data collection technique was determined by considering the site characteristics and the specific types of data required. Data was collected for up to 14 months at each location.
Several measures of effectiveness related to vehicle speed were analyzed to determine the effects of the DSFS as a function of sign size/type, lateral installation position, and activation location. The dependent variables for these analyses were selected to assess driver response to feedback messages and included:
- Speed at the point of curvature (i.e., curve entry)
- Speed 250-ft upstream of the point of curvature
- Speed 400-ft upstream of the point of curvature
FINDINGS
- DSFS systems located 250 to 400 ft upstream of exit ramps lowered curve entry speeds by 4.5 mi/h. The speed reductions measured at the third, sixth and 14th month after the installation indicated that the DSFS did not lose its effectiveness towards reducing curve entry speeds with time.
- DSFS was equally effective irrespective of the mainline speed limit or ramp advisory speed.
- Regarding sign size, 15” and 18” signs were found to be equally effective, and a prominent yellow boarder is recommended amidst visual clutter.
- The results also revealed that activation of the DSFS did not demonstrate any significant impact on the speeds of mainline vehicles.
- Finally, the effectiveness of DSFS was similar between system interchanges and service interchanges.
