Synthesis of Safety for Traffic Operations - Transports Canada

Intersection Control
TABLE 3.29: Study Site Characteristics for White Strobes to Supplement Red
Signal Indications
Site
Major Road
Minor Road
No.
No. Speed
Speed
Strobes/
Approaches limit ADT limit ADT
approach
(mph)
(mph)
1 1 4 45 11000 45 6300
2 1 4 55 21000 55 11000
3 2 4 40 9000 40 4400
4 2 3 45 14200 45 1100
5 2 4 55 9400 45 2500
6 1 4 45 11000 45 2400
TABLE 3.30: Crash Results for White Strobes to Supplement Red Signal
Indications
Site
Rear-end Angle Total
Before After CMF Before After CMF Before After CMF
1 4 4 1.00 13 8 0.62 19 20 0.95
2 2 7 3.50 1 5 5.00 3 15 5.00
3 4 4 1.00 8 3 0.37 13 7 0.54
4 6 6 1.00 4 1 0.25 12 9 0.75
5 0 0 1.00 1 1 1.00 2 2 1.00
6 3 6 2.00 12 15 1.25 15 28 1.87
All 19 27 1.42 39 33 0.85 64 81 1.27
Although the results of the analysis indicate that the strobe lights are likely detrimental to
overall intersection safety, no significant conclusions can be drawn because of several
flaws in the study design. First of all, the sites were not selected at random, there is a
likely regression-to-the-mean effect, which would suggest that the strobe lights were
actually more detrimental than the results would indicate. Secondly, the lack of a
comparison group means that other factors that may have influenced crash frequency
were not accounted for. Thirdly, exposure was not accounted for, and increased traffic
volumes would certainly play a role in increasing crash frequency. Lastly, the strobes
can only be expected to affect crash occurrence while they are flashing (i.e., during the
red phase). Reviewing all crashes, including those when the signal indication is green
would confound the results.
Sayed et al (1998)
Sayed et al (1998) researched the safety impacts of a revised signal head configuration at
10 intersections in British Columbia. The existing standard signal head (and the “before”
Page 38