In-Service Evaluation of Culvert Extensions

Project status

In progress

Start date: 07/01/17
End date: 12/31/18


Principal investigators:

About the research

Current design manuals direct designers to extend culverts to the edge of clear zone before connecting to the wing wall. However, in the case of large culverts, placing vertical drop-offs at the edge of clear zone is still concerning and may warrant protection. In these cases, since a barrier may already be warranted, there exists a potential cost savings by shortening the culvert and connecting to the wing wall just beyond the deflection of the barrier.

While introducing a barrier would replace one hazard with another, unprotected culverts/pipe openings account for an annual average societal cost of $33.5 million. Protecting these openings with barriers may be a cost savings by itself, but paired with the reduced construction cost by shortening the culvert, the combination may be even more beneficial.

The goals of this study are as follows:

  • Survey state DOTs to determine the height at which a vertical drop becomes a hazard even if placed outside the clear zone (as this height would translate to the opening size that should be protected, regardless of location)
  • Review crash rates for various culvert offsets and interaction with barriers for known locations
  • Develop benefit/cost per foot of culvert extension and relate to cost of barrier (both installation and presumed increased costs from crashes) to determine combinations of wing wall offsets and barriers for which shortening culverts is cost beneficial


Sponsor(s): Iowa Department of Transportation