Roadway-Design Question regarding "Posted" Speed versus "Design&q

[Knarf1Man]

Hi all.

We are designing a new public road and my question is regarding the correct/preferred method for using a "Design-Speed". All vertical & horizontal geometry for the new road will comply with AASHTO & State-DOT standards.

I've read/been-told that the Design-Speed is always higher than the "Posted-Speed". The AASHTO Green-Book doesn't really spell out any "rules-of-thumb" on the subject, only some gobbly-goop about using the 85th percentile of actual vehicle traffic-speeds (lol).

We would like to use a Posted-Speed of 30 mph. Due to existing topo/property constraints, we need to keep the Design-Speed as low as possible. We anticipate a low daily traffic volume under 2000 veh/day.

So ...... is a Design-Speed of 35 MPH ok? Or do we need to bump-it up to 40 MPH.

All comments/suggestions appreciated on this subject.

Thanks-In-Advance.

 

[ACtrafficengr]

You may want to wait for the next Green Book. The current issue of Public Roads has this to say:

[qoute]A Policy on Geometric Design of Highways and Streets. The next edition of this AASHTO publication, commonly referred to as the Green Book, will include information on design flexibility, rumble strips, and roundabouts. In terms of flexibility, the guide will highlight opportunities for highway designers to consider context when selecting design criteria. Applying a limited set of design values tends to favor one type of user over another -- cars, trucks, transit, pedestrians, or bicyclists -- and is not appropriate for every setting or type of road.[/quote]

http://www.fhwa.dot.gov/publications/publicroads/10septoct/05.cfm

Since this is a new road, you can choose a design speed appropriate for the road's terrain and function.

A new paragigm is design speed consistency rather than minimum design speed. This goes back to the classic problem of the long tangent followed by a 30 mph curve. Yes, the tangent meets the 30 mph design speed, but it still cause problems.

You could also look into the IHSDM software:

http://www.ihsdm.org/wiki/Welcome.

You can load your CAD design into the software, and it will analyze it for safety, design exceptions, design consistency, etc. Best of all, it's free.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928