Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
models predict the annual number of crashes on an arterial approach (both directions
included). Of the many variables that were found to influence crash occurrence, the type
of signal control was included. Adaptive signal control was found to be safer than
pretimed signal control, reducing crashes by 13% (CMF of 0.87).
Night-time Flash
Polanis (2002)
Polanis (2002) reported on the removal of red/amber night-time flashing operation from
19 intersections in Winston-Salem, North Carolina. Site selection information is sketchy;
the intersections are “not necessarily… high-crash locations”, but are described as
locations where the crash pattern indicated a safety benefit from night-time flash
removal. Target crashes are right-angle crashes that occur during the night-time flashing
operation. The results of the study are shown in Table 3.56.
Sixteen of the 19 intersections exhibited a statistically significant reduction in target
crashes at the 95% level of confidence. Polanis aggregates the results for all intersections,
which yields CMFs of 0.22 for target crashes, and 0.67 for all right-angle crashes.
There is no reason to believe that removal of night-time flashing operation would have
any measurable effect on right-angle crashes at other times of the day. Hence, rather than
measuring the safety impacts on total right-angle crashes, it is more informative to use
the non-target crashes (total right-angle crashes minus target crashes) as a control group.
If this is the case the aggregated data produces the statistics shown in Table 3.57.
These data indicate that the target crashes were reduced by 78%. However, the nontarget
crashes were also reduced, by 19%. Since the night-time flashing operation should
have had no effect on the non-target crashes, a 19% reduction in target crashes could
reasonably be expected without removal of night-time flash. Taking into account this
background crash reduction leads to a CMF for removal of night-time flashing operation
of 0.27, or a 73% reduction in right-angle crashes during the flashing operation.
It bears mentioning that the above study did not account for exposure. This is an
important consideration because, although traffic volumes are likely increasing and
omission from the analysis leads to under estimation of the safety benefits, the growth of
traffic during night-time flashing operation is likely significantly less than during the
remainder of the day. If this is indeed the case, then not accounting for exposure leads to
an over estimation of the safety benefits of night-time flash removal.
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