Relationship Between Volume-to-Capacity Ratios and Accident Rates

Abstract

The general relationships between hourly accident rates and hourly traffic volume/capacity ( v/c) ratios were examined. A 26 km (16 mi) segment of Interstate I-94 in the Detroit area was selected as the study segment. The v/c ratios were calculated from average hourly traffic volume counts collected in 1993 and 1994 from three permanent count stations. Accident rates were derived from hourly distributed number of accidents in the same 2 years. The correlation between v/c values and accident rates follows a general U-shaped pattern. The study of all observed accidents combined indicates that accident rates are highest in the very low hourly v/c range, decrease rapidly with increasing v/c ratio, and then gradually increase as the v/c ratio continues to increase. U-shaped models also explain the relationship between v/c and accident rates for weekdays and weekend days, multivehicle, rear-end, and property-damage-only accidents. On the other hand, single-vehicle, fixed-object, and turnover accidents, and accidents involving injury and fatality follow a generally decreasing trend with increasing v/c ratio. Traffic conflict is viewed as a major contributing factor to high accident rates observed in the high v/c range, whereas night conditions and driver inattention were identified as explanatory factors for the occurrence of high accident rates in the low v/c ranges.

A spatially disaggregate analysis of road casualties in England

“…The results for population density would also tend to confirm the finding that more congested areas result in relatively fewer casualties, although this is less likely to occur in areas with higher employment density, which may be equally congested. Some studies that have found more congested roads to be safer (Zhou & Sisiopiku, 1997) and suggests that policies of reducing congestion in urbanized areas may have unanticipated safety consequences (Shefer & Rietveld, 1997).…”

Application of Travel Time Reliability for Performance Oriented Operational Planning of Expressways

“…Zhou and Sisiopiku 18) found that the correlation between v/c ratio and accident rate follows a general U-shape pattern on the freeways of the United States. Chang, et al 19) as well as Hikosaka and Nakamura 20) observed the same tendency on Korean freeways and Japanese expressways, respectively.…”

Tool to Evaluate Safety Effects of Changes in Freeway Traffic Flow

“…There is strong empirical evidence of functional relationships between crash rates and traffic flow (Aljanahi, et al, 1999, Cedar and Livneh, 1982, Dickerson, Peirson and Vickerman, 2000, Frantzeskakis and Iordanis, 1987, Garber and Gadiraju, 1990, Gwynn, 1967, Hall and Pendleton, 1989, Jansson, 1994, Johansson, 1996, Jones-Lee, 1990, Maher and Summersgill, 1996, Newberry, 1988, O'Reilly, et al, 1994, Sandhu and Al-Kazily, 1996, Shefer and Rietveld, 1997, Stokes and Mutabazi, 1996, Sullivan, 1990, Sullivan and Hsu, 1988, Vickery, 1969, Vitaliano and Held, 1991, and Zhou and Sisiopiku, 1997. Nevertheless, the manner in which safety is improved by smoothing traffic flow is not well understood at this time.…”