St. Charles County Transportation Plan 2030 - East-West Gateway ...

Crash Rates
The crash rate is the best measure of safety on a given road segment and is calculated as follows.
R = (A x 100 x 1,000,000)
(365 days x T x V x L)
where:
R = crash rate in crashes per 100 million vehicle-miles (100MVM).
A = number of crashes reported during the analysis period.
T = time of the analysis period in years.
V = annual average daily traffic on the road segment.
L = length of the road segment in miles.
Table 18 lists the roads by functional class with their crash rate. It relates the number of crashes to
the length of the road and traffic volume. Table 19 ranks them by their crash rates. The road with
the highest was Rt. 94 from Rt. 370 through the city of St. Charles to I-70. It travels through the
urban core of the city and has many signals, driveways, and curb parking that create conflict points.
Other road segments at the top of the list include a few that have short lengths with relatively low
traffic volumes. However, they have many crashes because of numerous intersections and entrances.
The road segments with the lowest crash rates, those that are the safest, are I-70 and I-64 and all the
other freeways and expressways. Although they carry high volumes and have the most crashes per
mile, they are the safest for the amount of vehicles they handle due to their high design standards.
Severity Crash Rates
In order to further assess the safety of travel on the road segments, a severity crash rate was
developed. The crash rate formula described above was used but instead of using total crashes, the
number of fatal and disabling injury crashes were used. This allowed for examining the severity of
crashes on a road while taking into account its length and traffic volume. Table 20 shows the listing
of the roads and their ranking.
It was determined that the roads with the highest severity crash rates were those on two lane roads
mainly in rural parts of St. Charles County. First was Rt. 94 from Rt. D to Rt. DD, a two-lane road
with severe horizontal and vertical curves, and second was Rt. DD from U.S. 40/61 to Rt. D, which
also has severe curvatures. Roads with low severity crash rates, those that were safest, were the high
design facilities such as the interstates, freeways, and expressways.
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