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Section 6: Using the Safety Improvement Index

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Introduction

After verifying eligibility, the Texas Department of Transportation (TxDOT) Traffic Operations Division (TRF) subjects each eligible proposed highway safety project to a benefit-cost analysis. The formula used for this purpose is the Safety Improvement Index (SII).

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The SII Formula

In its most basic form, the SII is the ratio of the cost of crashes that have occurred at a location to the cost of constructing the proposed improvement. The SII incorporates adjustments to provide additional benefit for:

  • locations experiencing increasing traffic over the project life
  • improvements that will reduce maintenance costs
  • projects expected to have long service lives over which construction costs can be amortized.

The SII formula is as follows:

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
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Obtaining SII Data

Before calculating the Safety Improvement Index (SII), the “Proposed Corrective Action” described on the Safety Evaluation Report (SER) must be translated into “work codes.” Each SER may be defined by as many as three work codes. The Highway Safety Improvement Program (HSIP) Work Codes Table, found in Section 9 of this manual, provides the work codes that correspond to various descriptions of work. The table also provides associated definitions, reduction factors, service lives, applicable maintenance cost, and preventable crash codes.

The data necessary to calculate each project’s Safety Improvement Index (SII) can be obtained from the sources shown in the following table.

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Data Item

How It Is Obtained

R — Percentage Reduction Factor

NOTE: The reduction factor represents the percentage reduction in crash costs or severity that can be expected as a result of the improvement.

From the Highway Safety Improvement Program (HSIP) Work Codes Table, found in Section 9 of this manual.

NOTE: If the project is represented by more than one work code, Texas Department of Transportation (TxDOT) Traffic Operations Division (TRF) program administrators derive a composite reduction factor.

F — Number of fatal and incapacitating injury crashes

I — Number of non-incapacitating injury crashes

The HSIP Work Codes Table shows “Preventable Crash” codes. Preventable crashes are those with defined characteristics that may be affected by the proposed improvement as described by the work code. The codes correspond to numeric codes assigned in the Crash Records Information System (CRIS) to the indicated variable. Information is collected from the peace officer’s crash report and converted into a coded format. The Preventable Crash Decoding Table ( Section 10 of this manual) can be used to interpret the codes and determine the number of each type of crash. Three years of preventable crash data are used. The program call specifies the years used.

Cf — Costs of fatal and incapacitating injury crashes

Ci — Costs of non-incapacitating injury crashes

The average cost of each type of crash is based on the comprehensive cost figures provided by the National Safety Council. The program call provides the cost figures used each year.

L — Project service life

From the HSIP Work Codes Table found in Section 9 of this manual.

NOTE: If the project is represented by more than one work code, TRF program administrators base the project service life on the primary work.



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SII Results

A project with an SII greater than or equal to 1.0 is considered cost effective. Projects with an SII of less than 1.0 will not be considered for funding.

NOTE: The SII does not establish the need or lack of a need for a project. The SII formula compares costs of crashes to costs of construction; it provides no evaluation of the appropriateness of the type of construction.

The SII was designed as a comparison device for project prioritization and should not be used as a measure for independent projects.

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SII Calculator Available

To open an Excel-based program for calculating a project’s Safety Improvement Index (SII), click on the following link:

SII_calc.xls

NOTE: All fields are protected except for the input fields.

NOTE: “Cost” amounts subject to change on an annual basis.

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