Synthesis of Safety for Traffic Operations - Transports Canada

Traffic Signs
Apart from Arnott’s expressed concerns with the results of his study, there is also a high
potential for regression-to-the-mean effects to be overestimating the results. The sites
were likely selected because of an elevated incidence of crashes. Furthermore, time
trends and exposure have not been controlled for at any of the sites. For example, at
Site 1 the 22 non-target crashes (99 total crashes minus the 77 target crashes), while not
supposedly impacted by the treatment, decreased by 22% from the before to after periods.
This is almost identical to the 21% decrease observed in the target crashes group. One
cannot comfortably conclude that the traffic activated curve speed warning signs had any
significant impact on safety.
Zador et al (1987)
Zador et el (1987) completed a study to compare driver behaviour at horizontal curves
outfitted with chevrons, post-mounted delineators (PMD), and raised pavement markers
(RPM). The devices were independently installed at sites that varied systematically in
direction, degree of curvature, and steepness of grade. The study used before and after
data on lane placement and speed with a comparison group.
There were 51 sites located on two-lane rural roads. The treatments were installed in
accordance with the requirements of the FHWA MUTCD. All sites, including the
comparison sites, had edge line markings. The treatments being evaluated are as
follows:
• RPMs: 4”x4” amber markers installed on both sides of a double yellow
directional dividing line and throughout the curve. The RPMs were visible to
both directions of travel and were recessed into the pavement. Spacing was
typically 80 feet; along sharper curves where the 80 foot spacing did not result in
at least three RPMs being visible at all times, the spacing was decreased to 40
feet.
• PMDs: White, round delineators with a 3 inch diameter on metal posts were
installed on the outside of the curve. PMDs were visible to drivers from both
directions. Spacing was determined such that a motorist could view at least three
delineators simultaneously.
• Chevrons: 18” x 24” chevron signs mounted on the outside of the curve and
visible to drivers in both directions. Spacing was determined such that a motorist
could view at least three chevrons simultaneously.
Zador et al found that all of the treatments affected driver speeds at night. The
differences between the treatment type and the roadway alignment were few, and they
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