Synthesis of Safety for Traffic Operations - Transports Canada

Intersection Control
Sayed et al (1999)
Sayed et al (1999) examined the safety of providing advance warning flashers (AWFs) at
signalized intersections in British Columbia. A total of 106 intersections were used in the
analysis; 25 of which were equipped with AWFs. The treatment consists of a rectangular
warning sign that is equipped with two amber beacons mounted on either side, that
operate in alternate flashing mode. The signs are illuminated and erected overhead, but
positioned over the shoulder. The location of the sign and the onset of flashing operation
are in accordance with generally accepted Canadian practice.
The study methodology included the development of SPFs for intersections with and
without AWFs to determine the safety impacts of these devices. The use of properly
developed SPFs accounts for regression-to-the-mean. The SPFs were developed and
used in three different ways to evaluate the safety impacts (i.e., the form of the SPF was
constant among all situations but the researchers experimented with model development
by varying or holding constant certain parameters between SPFs). The results are shown
in Table 3.40.
TABLE 3.40: Safety Impacts of AWFs in British Columbia
Method
CMF
Total Injury+Fatal Rear-end
1 0.92 0.91 1.03
2 0.88 0.86 0.97
3 0.82 0.86 0.92
All three methods show remarkably similar results. Methods 2 and 3 did not produce
statistically significant results at the 95% level of confidence. Upon further investigation,
Sayed et al determined that the safety impacts of the AWFs were associated with the
minor street volume at signalized locations. The results of this more in-depth
investigation are shown in Figure 3.1. The results indicate that when minor street
volumes are relatively low, and main street volumes are high, the AWFs actually degrade
safety. A minor street volume of about 13,000 vehicles per day is required before the
AWFs provide a safety benefit for all major street volumes.
Page 46