California - Understanding posted/operating/design speed 1

[lgomez]

I have a project involving the re-design of an existing rural County roadway in Central California. The client would like us to re-design the roadway to the current Caltrans design guidelines, but maintain the existing posted speed limit of 45mph. The existing roadway has had a number of accidents and fatalities.
Question: What "design speed" should I be using? I have been told a "rule of thumb" that the design speed should be 5-10mph above the posted speed. My understanding is once the design speed is selected, all the horiztonal and vertical layout information is determined from the Caltrans Highway Design Manual.
Also, is it safe to say that re-design of an existing unsafe road to current Caltrans standards, while maintaining the existing posted speed, would imply that the new roady is considerably longer, wider, etc?

 

[francesca]

If you design it for faster than the posted speed, people will go faster than the posted speed. Don't build a runway unless you want people to fly.

There's a road in rural Tennessee with 10 ft shoulders/12 ft lanes and long tangents/gentle curves. The posted speed limit is 30 mph. The cops hide and ticket all day long and the town seems to be weathering the recession extremely well.

 

[jgailla]

I'm not a roadway guy, but the design speeds on roadway projects I've managed are normally 10 miles over the posted speed. This is in Florida, but I believe the AASHTO Green Book has guidance on the proper relationship between the design speed and posted speed.
Also, I've driven in about 35 states (mostly on the interstate), and everywhere, local or highway, it seems to be the common practice to drive 5 to 10 miles per hour over the posted speed.
francesca also makes a good point about excessively low speed limits with in relation to the "feel" of the road.

 

[civilman72]

General practice is to make your design speed 5-10mph above the posted speed.

But when re-designing an existing road, you will probably have limitations in following this general practice, particularly if there have been accidents/fatalities in the past. This could indicate that there are areas of the existing road that do not meet the minimum design requirements for the posted speed. Your engineering judgement will have to come into play when deciding how much re-work of the road will be required to meet the posted speed, and then possibly the design speed. You may have to be prepared to provide a cost-benefit rationale to justify your design decisions.

 

[ACtrafficengr]

What speed is appropriate for the neighborhood?

The paradigm on design speed is shifting. It used to be based primarily on speed limit and 85% speed. Starting with the 2000 Green Book, the definition changed.

If you design for 10 mph above the 85% speed, you can expect speeds to go up. Then 15 years later, you design for 10 mph over the 85%, and so on, and so on, and then you wonder why everyone is speeding.


"...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