#02 - Cross Walk Discussion - Laserfiche WebLink

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In the expressions given above, the parameters p1 and p2 represent proportions of pedestrian volumes at marked sites adjusted for slight differences in exposure times over which crash data were obtained. These results suggest that, in general, marked crosswalks are less safe than unmarked crosswalks on streets having more than two lanes, but that the two types do not differ significantly on streets with two lanes. Note that the analysis described above did not require adjustment for motor vehicle volume, since matched pairs of marked and unmarked sites typically were selected at or near the same intersection where vehicle volumes were similar. To investigate the relationship between other factors and combinations of factors on crosswalk pedestrian crashes, generalized linear regression models were fit to the data to predict crashes as functions of these variables. Consider a model based on pedestrian volumes (ADP); traffic volumes (ADT); and two indicator variables, one which indicates one or two travel lanes (L2), and the other which indicates three or four travel lanes (L4). The resulting model has the form (4) where E (Accsi) is expected pedestrian crashes at site i, yrsi is the number of years over which crash data was available for site i, and β0, β1, ... , β4 are parameters to be estimated. Models of this form were fit to data from marked and unmarked crosswalks separately. The models were fit by maximum likelihood methods using Procedure for General Models (PROC GENMOD) software, as developed by the SAS Institute. Crashes were assumed to follow a negative binomial distribution. Parameter estimates for these basic models are shown in table 2. Table 2. Parameter estimates for basic marked and unmarked crosswalk models. Marked Crosswalks Unmarked Crosswalks Parameter Estimate S.E.* p-Value Estimate S.E.* p-Value Constant ($0) -14.55 1.95 < .0001 -10.25 2.72 .0002 ADP ($1) .381 .065 < .0001 .602 .134 < .0001 ADT ($2) 1.006 .184 < .0001 .304 .258 .2388 L2 ($3) -.599 .328 .0678 -.066 .592 .9115 L4 ($4) .075 .247 .7608 -.208 .553 .7076 *S.E. = Standard Error For marked crosswalks, the results in table 2 show that expected crashes increased to a significant degree with both increasing pedestrian volume and increasing traffic volumes, with a much steeper increase for traffic volume. The lane variables compare two-lane roads with roads having five or more lanes, and three- or four-lane roads with roads having five or more lanes. The two-lane variable is marginally significant, while the three- or four-lane variable is not. The overall lanes effect (not shown) is significant (p-value of .0262). In subsequent models, a two-level lanes effect comparing two lanes with three or more is used. This variable is usually significant at a level of about .02. The results for unmarked crosswalks show the only statistically significant effect to be for pedestrian volume. Thus, expected crashes on unmarked crosswalks increased consistently with increasing pedestrian volumes (at a somewhat higher rate than that at marked crosswalks), but did not change consistently with increasing traffic volumes or with number of lanes. These results suggest that multilane streets with low traffic volumes might represent another subset of the data where marked and unmarked crosswalks might not differ significantly with respect to safety. This issue is addressed in more detail later in the report. 19