Where the minor road (i.e. relatively less important) overpasses the major road (e.g. freeway), the crossing should be nearly square and the alignment through the interchange should be as straight as possible. If the minor road must be on a curve, the radius should be as large as practicable. Reasons for this include:
- curved structures cost more than straight structures
- curved box girder bridges are difficult to be widened
- the curvature restricts sight distance from the exit ramp across the structure on the inside of the curve, and it is sometimes necessary to widen the bridge or provide a splay to meet sight distance requirements, and this is costly
- superelevation on a curved bridge
- can adversely affect sight distance on one of the ramps
- adds significantly to the difficulty and cost of construction
- may cause instability of trucks turning from exit ramps or onto entry ramps in some cases.