The proposed or existing roadway affects culvert cost, hydraulic efficiency, and alignment.

Information from the roadway profile and the roadway cross section should be obtained from preliminary roadway drawings or from standard details on roadway sections. If the culvert must be sized prior to the development of preliminary plans, a best estimate of the roadway section must be developed, and the culvert design must be confirmed after the roadway plans are completed.

Roadway cross sections normal to the centerline are typically available from highway plans. However, the required cross section at the stream crossing may be skewed with reference to the roadway centerline. To obtain this section for a proposed culvert, combine roadway plan, profile, and cross-sectional data as necessary.

Preliminary dimensions and features of the culvert should become evident when the desired roadway cross section has been evaluated or established. The dimensions may be obtained by superimposing the estimated culvert barrel on the roadway cross section and the streambed profile, which will establish the inlet and outlet invert elevations. The elevations and the resulting culvert length are approximate since the final culvert barrel size must still be determined.

The roadway embankment represents an obstruction to the flowing stream, much like a dam. The culvert is similar to the normal release structure, and the roadway crest acts as an emergency spillway in the event that the upstream pool (headwater) attains a sufficient elevation. The location of initial overtopping depends on the roadway geometry. Generally, the location of overtopping (roadway sag) should coincide as closely as possible to the location of the majority of flood flow under existing conditions. Since the roadway centerline profile may not represent the high point in the highway cross section, location of the actual low point is critical.