B/C methods Iowa DOT Texas

B/C analysis in highway safety – the Iowa DOT approach Following are actual applications submitted for Traffic Safety Funds. The applications include a B/C prepared by the applicants:  TSIP Application US 69 Curves by Randall.pdf 19th & MLK Corridor Safety Improvements--Signed tsipforms.doc.pdf

Below are the B/C sheets prepared for each submission: US 69 B-C.xls 19th & MLK North Section B-C.xls 19th & MLK South Section B-C.xls

Typically, revisions made to the B/C are: Eliminating crashes because they are not within the project or have no chance of being reduced by the improvement type (animal crashes) Changing the Crash Reduction Factor (CRF) to something more appropriate Changing the expected life of the improvement.

There are several other nuances. The first fatality is treated as a major injury so one rare event does not dominate the analysis. You must take care to only include crashes that match the CRF that is used. You can't use a factor that will reduce rear-end crashes by 40 percent and apply it to all crash types. (Same for severity.)’

If you want to use a CRF that only applies to severe crashes you also have to consider if that treatment may lead to an increase in less-severe crashes. (e.g., cable median barrier) The new HSM methodology takes this into account. For the Iowa DOT tour B/C sheet, you can combine multiple CRFs, but you have to use the formula provided; each CRF must apply to all crash types; and you must use the shortest design life of the various improvements. (again, the HSM provides more robust methodology to determine future benefits for multiple design lives.)

The Safety Improvement Index (SII) TXDOT application SII = B/C, where: S = annual savings in crash costs (equal to crash cost savings per year less annual maintenance costs) R = percentage reduction factor (see following subsection for explanation) F = number of fatal and incapacitating injury crashes (see following subsection for explanation) Cf = cost of a fatal or incapacitating injury crash (see following subsection for explanation) I = number of non-incapacitating injury crashes (see following subsection for explanation) Ci = cost of a non-incapacitating injury crash (see following subsection for explanation) Y = number of years of crash data M = change in annual maintenance costs for the proposed project relative to the existing situation Q = annual change in crash cost savings Aa = projected average annual Average Daily Traffic (ADT) at the end of the project service life Ab = average annual ADT during the year before the project is implemented L = project service life (see following subsection for explanation) B = present worth of project benefits over its service life C = initial cost of the project