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multiplicative relation between relative driver risk and rela- TABLE 3
tive road risk. UNDIVIDED VERSUS DIVIDED HIGHWAYS: COMPARISON
OF NATURALISTIC DRIVING EXPOSURE POINTS TO
TRAFFIC CONFLICT DATA
Event Type
ROAD SELECTION: DIVIDED VERSUS
Exposure (%) Traffic Conflict (%)
UNDIVIDED ROADS
Roadway Type (N = 1,072) (N = 907)
Recall the speed paradox presented and discussed in chapter Undivided 113 (10) 248 (38)
one. Statistics were presented showing that, perhaps contrary
Divided 959 (90) 559 (62)
to intuition and expectations, truck travel at speeds above
Total 1,072 (100) 997 (100)
50 mph was generally far safer than travel under 50 mph. A
principal reason underlying the speed paradox is that most
higher-speed truck travel is on divided highways, whereas
most lower-speed travel is on undivided roads. Further, most
highways) in the traffic conflict sample with those of the expo-
divided highways have limited access (i.e., entrance- and
sure points. Any condition overrepresented in the traffic con-
exit-ramps), whereas undivided highways are open-access.
flict sample can be considered a safety risk factor.
The safety advantages of divided over undivided highways
These comparisons demonstrate the disproportionate risk
are well known to road designers and others in road safety.
associated with driving on undivided highways. Table 3 com-
About 85% of large-truck crashes involve another vehicle, and
pares 1,072 baseline epochs (representing exposure) to 907
interaction among vehicles is greatest on undivided roadways.
traffic conflicts (crashes, near-crashes, and other incidents)
On undivided roads there are traffic signs and signals, crossing
from a long-haul truck naturalistic driving study (Hickman
traffic, stops and starts, turns, pedestrians and bicyclists, and
et al. 2005; Knipling et al. 2005). The percentage breakdowns
generally greater opportunities for distractions and other driver
are shown. Most divided roads in the study were Interstates or
mistakes. On divided highways, vehicles are all traveling
other freeways.
at about the same speed with minimal interaction and few
crossing-path events. Thus, divided highways have much
Only 10% of the driving was on undivided highways,
lower crash rates. Overall, Interstate highway fatal crash rates
but 38% of the traffic conflicts occurred on those roads. The
are about one-half those of non-Interstate arterial roads, and
majority of traffic conflicts occurred on divided highways
just one-third those of local roads (FHWA 2000). In eight
(62%), but the risk relative to exposure was much greater
studies cited in the FHWA website on crash modification fac-
on undivided roads. The odds ratio between undivided and
tors (www.cmfclearinghouse.org), the road design counter-
divided roads for traffic conflicts was calculated to be 5.3
measure "install roadway median" reduced crash rates by an
[(248/113) / (559/959) 5.3]. Figure 4 shows the same
average of 49%.
relationship graphically.
Harwood (2006) emphasized the limited-access feature of
In the LTCCS, 38% of CT crashes occurred on undivided
most divided highways in the following comments regarding
(including one-way) roads, a percentage identical to that
roadway design and CMV safety:
in the naturalistic driving study. Percentages were similar
for single-vehicle and multivehicle CT crashes. A larger
The lowest crash rates on our roadway systems are on limited-
access highways, for example, freeways and toll roads. Higher percentage of LTCCS ST crashes (55%) was on undivided
crash rates occur on multilane non-freeways where direct access roads. STs have somewhat higher crash rates per mile trav-
is permitted, including both multilane divided highways and eled than do CTs, reflective of their greater exposure on undi-
multilane undivided highways. Two-lane highways have the
highest crash rates. Across the mix of highway types, crash rates
vided roads. In 2008, 54% of large-truck fatal crashes and
differ by at least a factor of 3 or 4 between typical rural two-lane 52% of nonfatal crashes occurred on undivided highways
highways and rural freeways. (FMCSA Analysis Division 2010). No national exposure
estimates using the same definitions are available for com-
Naturalistic driving data provide a compelling testimony to parison to crashes, but there is no question that the exposure
the risks of driving on undivided versus divided roads. Traffic percentage is much smaller.
conflicts (including crashes, near-crashes, and other incidents)
captured by onboard sensors and videos are classified by their These statistics indicate that carriers could reduce risks
conditions of occurrence and other characteristics. In this case, by making concerted efforts to dispatch and route trucks on
the condition of interest is roadway separation (divided versus divided highways. For load pickups and deliveries, it may
undivided). In addition, researchers randomly select a large not be possible to reduce exposure significantly to undi-
sample of exposure observations, or "exposure points," repre- vided highways. Greater opportunities exist in trip planning
senting normal driving. These "exposure points" are also clas- between loading and delivery points. The statistics suggest
sified by conditions of occurrence. Researchers then compare that, when given a choice, trucks are safer on divided high-
the frequencies of conditions (e.g., divided versus undivided ways even if that means significantly greater mileage.
