The Impact of Speed Limit Increases on Fatal Interstate Crashes

Assessing the Impact of Speed Limit Increases on Fatal Interstate Crashes
Sandy Balkin, Ernst & Young LLP
J. Keith Ord, Georgetown University

Executive Summary

Introduction

Data Source

Methodology

Key Points

Charts and Tables
Fig 1:
Trend Component
for Rural Arizona

Fig 2:
Seasonal Component for Rural Arizona

Figure 3:
Irregular Component
for Rural Arizona

Figure 4:
Significance Level
of First Rural Limit Change

Figure 5:
Significance Level of Second Rural Limit Change

Figure 6:
Significance Level of Urban Limit Change

Figure 7:
Monthly Fatal Crashes

Figure 8:
Rural Seasonal Component Significance Level

Figure 9:
Urban Seasonal Component Significance Level

Table 1:
Significant Rural Changes

Table 2:
Significant Urban Changes

Table 3:
Predicted number of fatal crashes attributed to the speed limit increase on rural interstates.

Table 4:
Predicted number of fatal crashes attributed to the speed limit increase on urban interstates.
Executive Summary

The purpose of the study is to investigate the relationship between speed limits and traffic-related fatalities. Specifically, we sought to answer the question: Does an increase in the speed limit result in a higher incidence of fatal crashes?

We used data from the Fatality Analysis Reporting System (FARS), maintained by the National Highway Traffic Safety Administration (NHTSA). FARS provides monthly data on numbers of fatal crashes for each state, with separate counts for rural and urban interstates. We used data for the period 1975-98, 1998 being the last year for which data are available. Changes in interstate speed limits occurred primarily in 1987 (rural only) and in 1996 (both urban and rural).

We carried out the data analysis using a time series technique known as structural modeling. This approach enables us to partition a series into its trend, seasonal and irregular (or residual) components and to evaluate the impact of major interventions such as speed limit changes. Based upon a review of the past literature, we formulated the impact of a speed limit change as a one-time percentage increase in the number of accidents, after which the seasonal and trend patterns in the series would be expected to remain similar to those of past years. The analysis was performed for each state, separately for urban and rural interstates.

The principal conclusions from the study are as follows:

19 of 40 states experienced a significant increase in fatal crashes along with the FIRST speed limit increase on rural interstates.

10 of 36 states experienced a significant increase in fatal crashes along with SECOND speed limit increase on rural interstates.

6 of 31 states experienced a significant increase in fatal crashes along with the speed limit increase on urban interstates.

29 states exhibited significant seasonality on rural interstates.

18 states exhibited significant seasonality on urban interstates.

Although the results are statistically significant as noted above, the numbers in some states may be small.

The seasonal patterns probably reflect changes in the number of vehicle miles traveled (VMT), with peaks occurring during holiday seasons. Seasonal analysis is critical to understanding any changes in pattern, since unadjusted comparisons for a few months immediately before and after a change could be seriously in error. Our analysis allows comparisons to be made after proper adjustment for seasonal effects. Overall, increases were seen in some states following speed limit changes. These increases were predominantly on rural rather than urban interstates.

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