Benefit

FHWA national study finds intersection conflict warning systems reduce total crashes by 27 percent for two-lane at two-lane intersections

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.


06/01/2016
Minnesota; United States; Missouri; United States; North Carolina; United States


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%

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Source

Safety Evaluation of Intersection Conflict Warning Systems

Author: Himes, Scott; Frank Gross; Kimberly Eccles; and Bhagwant Persaud

Published By: USDOT Federal Highway Administration

Prepared by VHB for the USDOT FHWA

Source Date: 06/01/2016

Other Reference Number: Report No. FHWA-HRT-16-035

URL: https://www.fhwa.dot.gov/publications/research/safety/16035/16035.pdf

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Goal Areas

Safety

Typical Deployment Locations

Statewide

Keywords

None defined

Benefit ID: 2017-01133