A Full Bayesian Assessment of the Effects of Highway Bypasses on Crashes and Crash Rates

A common contention is that the construction of highway bypasses negatively impacts the economy of local communities by reducing pass-by traffic for businesses. However, as access to specific business account records is limited, this is difficult to quantify. Another common contention is that a reduction in crashes will be achieved. The actual impact of highway bypasses in the United States has not been assessed. This study seeks answers to the following questions with the use of a full Bayesian analysis: • Do bypasses in Iowa affect crash frequencies and rates on the bypassed highway? • Do bypasses in Iowa introduce a reduction of overall crash frequencies and rates, or do they merely shift crashes from the highways through the communities to the bypasses with no significant overall reduction? The results strongly suggest that the construction of bypasses in Iowa increases traffic safety both on the main road through town and on the sum of the main road and the bypass