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16 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.