E 543 Overpass Versus Underpass

Since cost is an important consideration, several preliminary layout plans including overpasses and underpasses should be made, comparing the advantages and disadvantages of each. The following factors should be examined and should influence the choice of using an underpass or an overpass:

1. Both the underpass and the overpass should fit into the existing topography, not only at the intersection but for the entire area to be utilized. This makes for a more pleasing appearance and reduces maintenance of extensive side slopes and pavements.
2. Both the underpass and the overpass require the use of open cuts or embankments, which are usually less costly than concrete walls or retaining wall construction. However, side slopes may add to the cost of right of way and may require perpetual maintenance.
3. An underpass generally creates greater drainage problems since a sump is created. The drainage facilities in sumps should be designed to remove surface runoff from a storm of 50-year frequency. This will usually require a major storm drain pumping plant. It may be necessary to provide an extensive storm drain system upstream from the underpass to reduce the storm runoff which must be pumped. This alternate solution is also very costly.
4. An underpass creates a problem with the network of subterranean pipes, utilities, and services which almost invariably exist in City streets. These must be rerouted or abandoned, which may involve additional expenses.
5. An overpass gives an advance warning to a person driving a vehicle that there is a change of grade ahead.  An underpass dips beneath the railway and the change in grade is not as readily apparent.
6. An overpass offers the best possibility for stage construction, both in the highway and in the structure.
7. An overpass requires a minimum vertical clearance of 7.5 feet greater than that of the underpass. This additional height means adding to the length of the vertical curve, which may result in a greater differential in grade between the approaching grade and existing improvements.  The greater grade differential usually means higher costs due to additional grading or wall construction, longer pavement lengths, and additional right of way.

Comparative costs of the alternate over-versus-under highway routes have indicated that most underpasses are less costly than overpasses. Previous studies have indicated that highway relocation is less costly than rail relocation.  However, it is incumbent upon the designer to investigate and consider all these factors, make an economic comparison of these alternatives from an engineering viewpoint, and arrive at the meet economical and acceptable solution for the project being considered.