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18 Murray et al. (2005) analyzed work zone crashes and sug- TABLE 4 gested truck-related improvements. The study found rear- TRAFFIC DENSITY: COMPARISON OF NATURALISTIC DRIVING EXPOSURE DATA TO RISK DATA end and sideswipe crashes to be among the most common Event Type scenarios. Work zone fatal crashes are more likely than non- Exposure (%) Traffic Conflict (%) work zone crashes to involve multiple vehicles. Nearly one- third of fatal work zone crashes involve a truck, although the Traffic Density (N = 1,072) (N = 914) study did not determine relative fault or principal causes. Heavy (C,D,E,F) 36 (3) 145 (17) Work zone crash countermeasures suggested by the study Medium (B) 258 (24) 216 (24) included crossroad rumble strips, driver feedback signing Light (A) 778 (73) 543 (59) (warning of excessive speed), highway advisory radio, and Total 1,072 (100) 914 (100) detection and warning of traffic queues. Thirteen percent of truck-crash involvements in the LTCCS occurred in work zones. Almost all of these involvements Increases in traffic density and travel times generate were in multivehicle crashes. Trucks were assigned the CR disproportionate increases in the number of proximal inter- (were at-fault) in 42% of these. Of all truck at-fault LTCCS actions among vehicles and associated crash risk. This is crashes, 11% occurred in work zones. Many of these were perhaps best seen in naturalistic driving data. Large-truck nat- rear-end crashes in which trucks struck cars, suggesting lia- uralistic driving methodologies and statistical findings relat- bility for trucks and their carriers. ing to traffic density and risk are similar to those presented earlier for undivided highways and for work zones. Table 4 In the safety-manager survey, avoiding construction zones shows exposure and traffic conflict percentages for different received an average rating of +1.4 on the -3-to-+3 Likert levels of traffic density from Hickman et al. (2005). As with scale. For other experts, the mean rating was +1.3. In both earlier examples, these are based on researcher observation cases, it was in the top half of safety practices but not among and classification of video views of surrounding traffic. A six- the very top. In safety-manager interviews, work zones were level classification scheme has been used to classify exposure cited several times as being among the risky road conditions points and conflicts. Light (A) means free-flowing traffic, to be avoided for safer operations. Two carriers described medium (B) means flowing with some restrictions (owing to specific efforts to avoid them. One carrier codes work zones the presence of other vehicles), and heavy (CF) means var- on its internal crash reports and has identified them as high- ious degrees of restricted traffic flow. Table 4 shows these risk areas. Another carrier provides drivers with daily state three groupings with heavy listed first as it is the highest-risk traffic alerts that include information on major work zones. condition. AVOIDING TRAFFIC In the table, notice the disproportionately high risk for heavy traffic density, equivalent risk for medium density, and The speed paradox (chapter one) and other evidence pre- lower risk for light traffic. The odds ratio of conflicts to expo- sented previously suggest that disrupted traffic flow elevates sure for heavy traffic (levels C, D, E, and F) compared with crash risk. Heavy traffic has become a dominant feature of lighter levels (A and B combined) is 5.9, indicating that inci- urban travel. The Texas Transportation Institute (TTI; http:// dent risk is about six times greater in heavy traffic. Also notice, publishes annual reports on urban traffic however, that the majority of conflicts still occurred in light congestion and its effects on mobility (Schrank and Lomax traffic, even though relative risk was lowest. 2009). In 2007, Americans lost 4.2 billion hours to urban con- gestion. This was a small reduction of about 1% from the pre- About half of all LTCCS truck-crash involvements occurred vious year, but was still more than five times the urban delay on urban roads, although only 28% were cited as having a 25 years ago. Across the United States, delay has increased in "traffic factor." LTCCS trucks were at-fault in 45% of their all types of urban areas, whether relatively small, medium- multivehicle crashes in urban areas, versus only 33% in rural sized, or large. In larger urban areas, free traffic flow occurs areas (Knipling and Bocanegra 2008). Trucks were also more reliably only between the hours of 9:00 p.m. and 5:00 a.m. In likely to be at-fault in crashes where traffic was a factor, per- 1982, peak morning congestion lasted about 75 min, from haps related to blind zones around trucks. about 7:30 a.m. to 8:45 a.m. Equivalent congestion now lasts almost 3 hours from about 6:30 a.m. to 9:15 a.m. For evening In the project survey, both safety managers and other peak hours, 1982's 90-min peak, between about 4:00 p.m. experts were asked to rate the driving practice "Avoid urban and 5:30 p.m., is now seen for twice as long, between about rush hours and other heavy traffic situations." As with other 3:30 p.m. and 6:30 p.m. The Travel Time Index is the ratio practices, they rated the safety value of the practice on a seven- of travel time in the peak period to travel time at free-flow point Likert scale, from -3 to +3. The practice received a conditions. Since 1982, the index has risen steadily from 1.09 mean rating of +1.7 from safety managers, making it one of to 1.25. That means that urban travel times during peak hours the highest-rated practices. The other-expert mean rating of are 25% slower than during free-flowing conditions. +1.2 was near the middle of the 11 practices rated.