Modeling crash-flow-density and crash-flow-V/C ratio relationships for rural and urban freeway segments

Road Traffic Congestion and Crash Severity: Econometric Analysis Using Ordered Response Models

“…Temporal effects were also considered. For single-vehicle crashes, they found a negative-exponential relationship with the density (volume/capacity ratio), meaning that the crash rate is highest at a low volume/capacity ratio, but this is not fully consistent with the study by Lord et al (2005) who conducted the analysis on the relationship among crash, density (vehicles per km per lane) and v/c ratio. They found that with v/c ratio increasing, fatal and single-vehicle crashes decreases at some point, and crash rates follows a U-shape relationship.…”

The statistical analysis of crash-frequency data: A review and assessment of methodological alternatives

“…sense because multiplying traffic flow and segment length gives a traditional exposure measure (vehicle-miles traveled). However, similar to traffic flow, some researchers have found a non-linear relationship to exist between crashes and the length of a segment (e.g., a doubling of segment length more than doubles the crash frequency) while others have argued that exposure should be linear Lord et al, 2005a). Most likely, conflicting and counterintuitive findings with regard to exposure could be pointing to unobserved heterogeneity and possibly other specification problems.…”

“…10 Most count-data models assume that explanatory variables influence the 10 The non-linear effect that explanatory variables have on crash-frequencies can be revealing. For example, some researchers have used traffic flow as a measure of exposure and found the relationship between crashes and traffic flow to be decreasing (Tanner, 1953;Mahalel, 1986;Hauer, 1997;Persaud and Nguyen; implying a potentially contentious finding that the crash risk per unit of exposure becomes smaller as traffic flow increases (Maher et al, 1993;Lord 2002;Lord et al, 2005a). In a similar vein, segment length has also been used as a measure of exposure because researchers have noted that the probability of observing a crash tends to be smaller on shorter roadway segments and higher with longer segments.…”

“…13 12 Usually the overdispersion parameter or its inverse is assumed to be fixed, but recent research in highway safety has shown that the variance structure can potentially be dependent on explanatory variables Hauer, 2001;Lord et al, 2005a;Cafiso et al, 2010b).…”

The effects of area-wide road speed and curvature on traffic casualties in England

“…Much research has been conducted to identify the various factors affecting road casualties (see Shankar et al, 1995;Shefer and Rietveld, 1997;Abdel-Aty and Radwan, 2000;Ivan et al, 2000;Noland and Oh, 2004;Lord et al, 2005;Aguero-Valverde and Jovanis, 2006;and Kim et al 2006). These factors are related to road infrastructure, traffic and socio-demographic characteristics, land use and the environment.…”