Comparison of Alternatives

Safety performance functions, development (calibration, crash modification factors [Part C], crash proportions), IHSDM and other software

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Comparison of Alternatives

Postby Srikanth » Thu Jul 14, 2011 12:28 pm

Footnote 3 of the table 6 (page 11) of the below link states, "3. Under the 5-lane scenario, the corridor has more capacity; therefore more regional traffic is drawn to this corridor. The decrease shown is for overall crashes, so a normalized analysis would show a slightly greater decrease."

I suppose the "overall crashes" is meant to state that FI & PDO are included for those particular intersection/segments. Guidance on the "normalized analysis" and some explanation would be helpful.

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Re: Comparison of Alternatives

Postby mdimaiuta » Thu Jul 21, 2011 1:59 pm

The crash frequency for the 5-lane alternative (Alternative 2) is 13% less than that of the No-Build scenario (25.8 crashes/year compared to 29.6 crashes/yr), even though the AADT’s for the 5-lane scenario are greater. (Note that the AADT increase for the 5-lane alternative is reflected in the part of the footnote that states, “…therefore, more regional traffic is drawn to this corridor.”)

I believe that a “normalized analysis” is one in which AADT is factored in, resulting in a metric like crashes/million-vehicle-miles/year (as opposed to just crashes/year). In that case, the percent decrease in crashes predicted for the 5-lane alternative as compared to the no-build scenario would be even greater. So, the footnote is stating that the reported 13% decrease in crashes for the 5-lane section is conservative; in reality, when traffic volumes are considered, the 5-lane section would result in an even greater decrease.
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