Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
include only the crashes that occurred during the same months as was available in the
“after” period.
The results of the Tribbett et al study are shown in Table 4.7.
Crash
Type
Casualty
crashes
PDO
crashes
All
crashes
TABLE 4.7: Safety Effects of Dynamic Curve Warning Sign Systems
Period
Site
1 2 3 4 5
All
Before 2.0 .4 0 0.4 0.4 3.2
After 2 0 2 0 0 4
CMF 1.00 --- --- --- --- 1.25
Before 1.4 1.2 0.8 1.2 1 5.6
After 1 2 2 0 0 5
CMF 0.71 1.67 2.50 --- --- 0.89
Before 3.4 1.6 0.8 1.6 1.4 8.8
After 3 2 4 0 0 9
CMF 0.88 1.25 5.00 --- --- 1.02
The “after” period in the Tribett et al study is too short to be representative of the stable
long-term crash record that might be expected at these sites post-sign installation.
Furthermore, exposure has not been accounted for in the analysis.
Close-following Warning Signs
Helliar-Symon and Ray (1986)
Helliar-Symon and Ray (1986) in a follow-up to an early study examined the safety
impacts of active warning signs for following too close. The treatment was a roadside
sign that displayed the message “Following Too Closely” or “Too Close Move Apart”
when the gap between vehicles was less than 1.6 seconds. The sign was blanked out
when not in use; the active message was supplemented with four flashing amber beacons
(one at each corner of the rectangular sign) that flashed in an alternating pattern. The
message was visible from the detector that measured the gap between vehicles, was
illuminated as soon as an inadequate gap was detected, and remained lit for two seconds.
The study methodology was a before-after analysis using a control group. Treatment and
control sites were selected on the basis of a high number of close-following crashes, and
for their suitability to accept the device installation. The metric used is injury-crash
frequency, using two years of before data and four years of after data at three sites. The
crash frequency was measured downstream of the sign installation, where the impact is
likely to be realized. Either separate sites, or road sections upstream of the sign were
used as control sites. The aggregated crash records from all three sites are shown in
Table 4.8.
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