National Demonstration Project Evaluates two types of Dynamic Speed Feedback Signs on Curves.
Arizona, United States
Florida, United States
Iowa, United States
Ohio, United States
Oregon, United States
Texas, United States
Washington, United States
Evaluation of Dynamic Speed Feedback Signs on Curves
Summary Information
The Center for Transportation Research and Education at Iowa State University conducted a national demonstration project to evaluate the effectiveness of two different Dynamic Speed Feedback Signs (DSFS) on reducing speed and crashes on curves on rural two-lane roadways.
- One type of sign displayed vehicle speed (referred to as the speed display sign)
- The other type of sign displayed a curve warning (referred to as the curve display sign)
Both signs were set to activate only when drivers exceeded the 50th percentile speed.
Methodology
The 7 participating states (Arizona, Florida, Iowa, Ohio, Oregon, Texas, and Washington) found 51 viable sites with an existing speeding problem. Signs were installed at 22 treatment sites between July 2008 and April 2010, while the remaining 29 sites served as control sites for crash analysis.
Speed and volume data were collected from the treatment sites at three points using automatic traffic recorders (pneumatic road tubes).
- Approximately 0.5 miles upstream from the curve.
- At the point of curvature (PC) where the sign was installed.
- At the center of curve (CC).
Data were collected at the three points for each of the 22 sites 1 month before installation of the signs and at 1, 12, and 24 months after installation. The data were collected over a period of 2 years to assess whether habituation occurs (i.e., drivers become immune to treatments and stop responding). For the crash analysis, crash data were collected for up to 4 years before and up to 3 years after installation of the signs. Crash data were evaluated for several different scenarios, including the following:
- Total crashes for both directions of travel.
- Total crashes by direction.
- Single-vehicle (SV) crashes for both directions of travel.
- SV crashes by direction.
Findings
- In general, larger decreases in speed were noted for the speed signs than for the curve signs, although differences were not statistically significant.
- On average, most sites had decreases in mean speeds, with decreases up to 10.9 miles per hour (mph) noted for both the point of curvature (PC) and center of curve (CC). Most sites experienced changes in 85th percentile speed of 3 mph or more at the PC, with the majority of sites having a decrease of 2 mph at the CC.
- 29.9 percent fewer vehicles exceeded the posted or advisory speed by 10 mph or more after the signs had been in place for 1 month.
- There were 35 percent fewer crashes per quarter for all vehicles in the direction of the sign, and 49 percent fewer crashes per quarter for just single-vehicles in the direction of the sign.
The following table shows the results of the average change in speeds across all sites at the PC:
|
1 Month |
12 Months |
24 Months |
||||||||
|
All Sites |
Curve Sign Sites |
Speed Sign Sites |
All Sites |
Curve Sign Sites |
Speed Sign Sites |
All Sites |
Curve Sign Sites |
Speed Sign Sites |
||
| Average Mean Speed (mph) | -1.82 | -1.68 | -1.95 | -2.57 | -2.47 | -2.66 | -1.97 | -1.99 | -1.96 | |
| Average 85th Percentile Speed (mph) | -2.19 | -1.90 | -2.45 | -2.86 | -2.40 | -2.70 | -2.17 | -2.00 | -2.30 | |
| Average Percent Change in Fraction of Vehicle Exceeding Posted Speed | 5 mph | -11.8% | -9.8% | -13.7% | -18.6% | -22.1% | -15.0% | -19.8% | -27.1% | -13.3% |
| 10 mph | -29.9% | -30.4% | -29.4% | -34.4% | -36.5% | -32.2% | -29.3% | -42.5% | -17.7% | |
| 15 mph | -36.3% | -39.4% | -33.5% | -36.2% | -27.3% | -45.2% | -29.6% | -42.5% | -18.2% | |
| 20 mph | -28.5% | -29.6% | -27.6% | -49.8% | -46.1% | -53.5% | -30.0% | -42.6% | -18.7% | |
The following table shows Before and After decreases in crashes:
| Site |
Before (crashes/quarter) |
After (crashes/quarter) |
Change (crashes/quarter) |
|||
|
Total |
SV |
Total |
SV |
Total |
SV |
|
|
Crashes for both directions |
||||||
| Control | .48 | .38 | .40 | .31 | -.08 (-17%) | -0.07 (-19%) |
| Treatment | .55 | .45 | .33 | .24 | -.0.22 (-40%) | -0.21 (-47%) |
|
Crashes in direction of sign or outside of curve |
||||||
| Control | 0.28 | 0.22 | 0.30 | 0.22 | 0.02 (+9%) | -0.01 (-4%) |
| Treatment | 0.35 | 0.29 | 0.23 | 0.15 | -0.12 (-35%) | -0.14 (-49%) |
SV - Single Vehicle
