In an effort to reduce traffic in central business districts, bypasses have been built to provide through traffic a circumferential route and also to ease the burden of traffic in downtown areas. In many cases, though, bypasses have created a serious accident potential because they were built with at-grade intersections and no access control. This has led to commercial developments along the bypass and congestion at major intersections. When congestion increases at an intersection, traffic signals must be installed; this increases vehicle operating and time costs and creates a rear-end collision potential. In some cases, the bypass circumvents a relatively small town where congestion is not an immediate problem, but there is still an accident problem at the at-grade intersections. Bypasses have been built in this manner because of the high initial cost of grade-separated interchanges and access control. Retrospective analyses of several bypasses were undertaken to determine if accident cost savings along with time and operating cost savings would have justified higher initial costs of interchanges. Such analyses may serve to justify needed improvements and to guide design decisions on future facilities.
The high initial cost of an interchange limits its use to those locations where the required expenditure can be justified. The conditions to be considered in reaching a rational decision are the applicable warrants:
- a freeway development,
- elimination of bottlenecks or spot congestion,
- elimination of hazards,
- site topography,
- road-user benefits, and
- traffic volume warrant.
On urban bypasses, Warrants 2, 3, 5, and 6 could be considered applicable in most cases. This study will be concerned only with Warrants 3 and 5.
When considering highway improvements, it is essential to view the investment as a business enterprise in which the economic costs of highways are matched by future economic benefits to the state, community, and individual user. Since the motor vehicle itself accounts for about 88 percent of the total cost of highway transportation, with highway construction accounting for the remaining 12 percent, the highway designer must consider the effects of highway design on the running cost of motor vehicles. The principal benefits of highway improvements accrue to those who travel the highway. These benefits reach the road user primarily through the operating cost of motor vehicles, reduction in highway accidents, and reduction in travel time. Such market factors are those on which a dollar value can be placed. There are also the nonmarket consequences of comfort and convenience.
Digital Object Identifier
Agent, Kenneth R., "At-Grade Intersections versus Grade-Separated Interchanges (An Economic Analysis of Several Bypasses)" (1975). Kentucky Transportation Center Research Report. 1090.