View Q&A

One common approach when addressing an intermediate size culvert with a diameter over 36” is to extend the culvert outside the clear zone.  Culverts extended in this way do not warrant shielding.  However, a hazard like a culvert with a 6’ vertical drop off or a water hazard just outside the clear zone may not be the best alternative.  Providing a barrier in situations like this is typically left to “engineering judgement”.  The Roadside Design Guide references factors such as the frequency of other objects located at the edge of the clear zone, facility speed and access control as considerations.  Another subjective thought is - will the impact with the hazard be worse than the impact with a barrier.  Is there any guidance available for hazards that may be outside the clear zone but where a barrier should be installed?

What are your thoughts?

We can understand the desire to further shield items like culverts that extend beyond the clear zone. As you noted, the clear zone is a concept based on providing a safe, recoverable area adjacent to the roadway for most departure conditions. However, there is always the potential for errant vehicles to extend beyond the clear zone and interact with hazards outside of that area. The question largely becomes an issue of practicality. DOTs cannot provide safety for an infinite lateral extent beyond the travel way, but they do want to provide a reasonable amount of safety. The clear zone is currently the best guidance for that.

The original clear zone guidance was developed to encompass around 80% of errant vehicles. That guidance has been updated over time adjust for slopes, ADT, and other factors.

Recent research by NCHRP and our colleagues at TTI looked at this issue to some degree in a study looking at traversable roadside slopes in NCHRP Report 1097 (see attached). In that study, TTI developed curves for the probability of A+K outcomes for varying encroachment conditions and lateral offsets. This data confirmed that the probability of severe impacts declines as the lateral offset of the hazard increases. It may help better define your concerns for potential impacts outside of your defined clear zone. The study also provides updated clear zone guidance that considers the roadside grading, hazard spacing, and curvature. It may help you better understand and define the culvert issue you are describing. The guidance is risk based.

As an alternative to be more conservative, one could consider making the ends of the culverts outside the clear zone traversable through the use of traversable slopes and culvert grates. While not required, it may provide some level of additional safety. However, the level of increased safety would need to be justifiable for the increased cost.

Hopefully, that discussion provides some insight into your questions. We would be happy to discuss it further if you would like.

Thanks!