FHWA pooled fund study evaluates the safety effectiveness of the use of an intersection conflict warning systems (ICSW) through scientifically rigorous crash-based studies in Minnesota, Missouri and North Carolina.
Minnesota, United States
Missouri, United States
North Carolina, United States
Safety Evaluation of Intersection Conflict Warning Systems
Summary Information
FHWA organized a pooled fund study of 40 States to evaluate low-cost safety strategies as part of its strategic highway safety effort. One of the strategies selected for evaluation in three states was intersection conflict warning systems (ICWSs), which are intended to reduce the frequency of crashes by alerting drivers to conflicting vehicles on adjacent approaches at unsignalized intersections.
Methodology
Geometric, traffic, and crash data were obtained for fourlegged, rural, two-way stop-controlled intersections with ICWS installations in Minnesota, Missouri, and North Carolina. Each State also identified approximately 30 reference sites without ICWS installation for four-legged intersections with two lanes on the major route and 30 reference sites without ICWS installation for four-legged intersections with four lanes on the major route.
- MnDOT provided crash data for installation and reference intersections from 2006 to 2012
- MoDOT provided crash data for the installation and reference intersections from 2000 to 2012
- NCDOT provided crash data for the installation and reference intersections from 1992 to 2012
Installation practices current at the time of this study used warning signs on the major approaches alerting motorists with the message "VEHICLE ENTERING WHEN FLASHING" (VEWF), "CROSSING TRAFFIC WHEN FLASHING," or "WATCH FOR ENTERING TRAFFIC." Signs on the minor approaches alerted entering motorists with "TRAFFIC APPROACHING WHEN FLASHING," "LOOK FOR TRAFFIC" (with yellow light-emitting diode (LED) arrow-shaped flashers), or visual graphic displays.
Application practices varied by state.
- Minnesota used a variety of post-mounted signs, including static signs with flashers, blank-out signs with flashers, visual displays, and signs with LED arrows indicating the direction of conflicting vehicles.
- Missouri used post-mounted static signs with flashers exclusively.
- North Carolina used static signs that were a mix of post mounted and overhead where the overhead signs were installed at the intersection. The results were combined to further draw inferences on the overall effect of ICWSs.
To account for potential selection bias and regression-to-the-mean, an empirical Bayes (EB) before–after analysis was conducted. The analysis also controlled for changes in traffic volumes over time and time trends in crash counts unrelated to the strategy.
Findings
The table below provides the estimates of expected crashes in the after period without installation, the observed crashes in the after period (with strategy), and the estimated crash modification factor (CMF) for all crash types considered. Results are provided separately for each State as well as for all States combined.
Aggregate analysis results for Minnesota
| Two-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
23.00 | 11.13 | 6.49 | 7.62 | 2.89 |
| Count of crashes observed in the after period (with strategy) |
20 | 6 | 13 | 0 | 3 |
| Estimate of CMF | .856 | .225 | .618 | N/A | .588 |
| Four-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
21.5 | 8.27 | 13.8 | 1.09 | 1.99 |
| Count of crashes observed in the after period (with strategy) |
16 | 9 | 9 | 1 | 2 |
| Estimate of CMF | .737 | 1.052 | .642 | .811 | 1.003 |
Aggregate analysis results for Missouri
| Two-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
47.08 | 16.18 | 25.42 | 5.74 | 3.71 |
| Count of crashes observed in the after period (with strategy) |
37 | 18 | 20 | 4 | 3 |
| Estimate of CMF | .777 | 1.088 | .771 | .642 | .0801 |
| Four-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
164.26 | 84.06 | 75.42 | 19.51 | 46.00 |
| Count of crashes observed in the after period (with strategy) |
119 | 47 | 61 | 16 | 28 |
| Estimate of CMF | .719 | .554 | .799 | .778 | .594 |
Aggregate analysis results for North Carolina
| Two-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
842.71 | 488.25 | 490.26 | 87.1 | 122.25 |
| Count of crashes observed in the after period (with strategy) |
613 | 338 | 387 | 39 | 110 |
| Estimate of CMF | .727 | .691 | .788 | .444 | .897 |
| Four-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
278.74 | 163.86 | 206.25 | 12.47 | 37.52 |
| Count of crashes observed in the after period (with strategy) |
250 | 156 | 182 | 16 | 23 |
| Estimate of CMF | .893 | .947 | .877 | 1.224 | .595 |
Aggregate analysis results for combined States
| Two-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
912.79 | 515.56 | 522.17 | 100.46 | 128.84 |
| Count of crashes observed in the after period (with strategy) |
670 | 362 | 420 | 43 | 116 |
| Estimate of CMF | .733 | .701 | .803 | .425 | .898 |
| Estimated reduction in crashes | 27% | 30% | 20% | 58% | 10% |
| Four-Lane at Two-Lane | Total | Fatal and Injury | Right-Angle | Rear-End | Nighttime |
|---|---|---|---|---|---|
| EB estimate of crashes expected in the after period without strategy |
464.50 | 263.56 | 295.47 | 33.07 | 85.52 |
| Count of crashes observed in the after period (with strategy) |
385 | 212 | 252 | 33 | 53 |
| Estimate of CMF | .827 | .802 | .850 | .973 | .612 |
| Estimated reduction in crashes | 17% | 20% | 15% | 3% | 39% |
