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20 about 2.1 (0.75 / 0.59 ÷ 0.25 / 0.41 2.1), indicative of greater Although its emphasis was not on traffic safety, a large risk during daytime. A time-of-day function based on the urban pilot test on truck deliveries has demonstrated huge time same data found the lowest incident rates to occur during the and cost savings from shifting day deliveries to nighttime. The overnight hours, whereas the highest were during the after- Research and Innovative Technology Administrationfunded noon hours (Knipling et al. 2005). pilot test arranged for participating carriers to make off-hour deliveries, instead of their regular day deliveries, to retailers Naturalistic driving studies may be challenged, however, and other receivers in New York City. The pilot test found that based on the concept that they capture many more non-crashes off-hour deliveries increased travel speeds by up to 75% and (e.g., hard-braking events) and very minor crashes (e.g., curb reduced unloading times at receiver sites by about 70%. It also strikes) than serious crashes. Based on a review of naturalistic reported a sharp reduction in parking tickets and fines, which driving data, crash data, and two different mileage by time-of- for daytime deliveries averaged more than $1,000 per month day exposure sources, Knipling (2009) reached the following per truck for participating carriers (NYC DOT 2010). The draft tentative conclusions regarding large-truck crash rate by time- project report by Rensselaer Polytechnic Institute (Holguin- of-day: Veras et al. 2010) gave no comparative crash data, but sug- gested that crash rates are lower with off-hour deliveries. · Overall, the large truck fatal crash rate per VMT appears Driver feelings of safety may be less, however, because of their to be roughly constant across the 24-hour day. personal safety concerns about night deliveries in a large city. · Nighttime fatal crashes are more likely to involve driver fatigue or alcohol use (by other motorists), whereas day- time fatal crashes are more likely to involve traffic inter- AVOIDING ADVERSE WEATHER action errors. · Nonfatal injury and property-damage-only crash risks Adverse weather is an obvious source of risk in driving and, are generally higher during the daytime hours and lower when extreme, can be a direct cause of crashes. A U.S.DOT at night. report (Rossetti and Johnsen 2008) argues that the role of · The hours between 6:00 p.m. and 2:00 a.m. appear gen- adverse weather in truck crashes is actually increasing relative erally to be the safest travel times for large trucks. to the overall truck-crash problem. That is because weather- · Overall crash risk rises in the early morning hours after related fatal truck crashes have declined less slowly over the 4:00 a.m. owing to the "one-two punch" of a circadian past decades than have non-weather-related fatal crashes. low period and morning rush hour traffic. There are a large number of different weather and climate fac- tors that can affect CMV transport safety. Some of these are In contrast to these findings, project survey respondents listed in the textbox. strongly favored day driving over night driving. The follow- ing are the mean Likert scale ratings for questions on this The percentage of large-truck crashes affected by adverse topic. The seven-point Likert scale for these questions ranged weather depends on the criteria used for "weather-related." In from -3 (Strongly Reduces Fleet Safety) to +3 (Strongly 2008, about 15% of truck crashes occurred during rain or other Improves Fleet Safety). "non-clear" weather condition. About 19% of fatal crashes and 22% of nonfatal crashes occurred on wet surfaces (FMCSA · Maximize day driving to avoid driver fatigue and other Analysis Division 2010). In the LTCCS, where causal factors nighttime risks: were more closely scrutinized, 14% of truck crash involve- Safety managers: +1.5 Other experts: +1.2. · Maximize night driving to avoid daytime traffic: Weather Effects on CMV Transport Safety managers: -0.4 Other experts: -0.7. Rain--loss of traction and control, delays Snow and ice--delays, loss of traction and control, tire damage from chains, ice on tops of vans Larger carriers are more likely to analyze their crashes in Thunderstorms and tornadoes--direct damage, relation to exposure factors such as time-of-day. For example, impaired visibility, loss of control case study Carriers C, D, and E all conduct such analyses. Temperature extremes--stresses on vehicle components, Several interviewees regarded overnight driving, particularly perishable cargo in the early morning hours, as more risky than day driving. High winds--vehicle instability and blowovers, especially vans More definitive research on this issue is needed because of Wet pavement--loss of traction and control, road spray the contradictory findings and because of the potential safety Hurricanes--direct damage, road closures benefits of reliable guidance on this issue. This research might Flooding--road closures, weak braking include fleet-based studies in which both crash incidence and Slides (snow, mud, rock)--collisions, delays exposure can be classified by hour-of-day, or studies of free- Source: Rossetti and Johnsen (2008). ways or major trucking lanes.