×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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.  

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

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources