How do you determine Curb dimensions based on Posted Speed?

[jptmenson]

I read curb height can be determined based on posted speed and not on design speed - how do you do that? References to specific standards will be helpful. Thanks

 

[ACtrafficengr]

You should use the standards preferred by the agency that will own and maintain the street.

Other than that, the main concern with curb selection is vaulting or tripping when struck at speed, especially if used with guide rail. If you want to use posted speed, you'll need to show that the post-construction prevailing speed will be in line with the agency's curb policy or the AASHTO Roadside Design Guide.

For example, a 2009 Connecticut State University study showed that roads with curb and sidewalk had lower travel speeds than curbless roads with similar geometry and context.

You could use the Design Consistency Module of the Interactive Highway Safety Design Model (IHSDM) to predict prevailing speed. I don't know if it is calibrated well enough to use it to make design criteria decisions such as this. The main emphasis of the module is the effective design speeds of contiguous road segments should be similar. Crashes tend to occur at places where the effective design speed suddenly drops.

As an aside, the concept of design speed is changing. As well as consistency, the new thinking is the design should foster speeds compatible with the road's context (residential, commercial, rural, etc). The old practice of setting it at at the posted speed or prevailing speed + 5 leads to speeding in urban and suburban contexts, not to mention higher project costs due to right-of-way takings, etc.