The 2001 Green Book, citing a 2000 NCHRP report, does not significantly change design side friction from the traditional values. This choice conflicts with the new MUTCD and is an exception from the acceptance of other recommendations compiled in Transportation Research Record 1701: Design Speed, Operating Speed, and Sight Distance Issues.
TRR 1701 included the paper pointing out that crest sight distance standards were too conservative. It also included a paper by Raymond Krammes pointing out that design speed as applied in the United States borders on a meaningless number.
"There seems to have been a change in emphasis from design speed as a speed which might be expected from driver behavior, to a speed which is 'safe' from the designer's point of view," he writes, quoting an Australian report.
SUV Driver Shot 4229223rem BS. If you haven't seen Mercedes driving 10 mph and-or cruising in the left lane you aren't looking. As...
We have a population of cookbook engineers who design roads in ignorance of the way drivers will use them. Drivers do not follow the Green Book side friction values. They did 60 years ago. Vehicles changed. Drivers changed. Standards remained the same. So what does the design speed mean if it isn't the speed at which people actually drive?
SUV Driver Shot 4230Hopefully someone will start shooting drivers out in my area. Can't believe the excessive stupidity I noted when going out for dinner a few...
Auto insurance settlementI'm sure many people have had the problem of being told "you car is totaled, we'll only pay you $loveX dollars, use it to buy another car that's the...
Engineers are encouraged to use "design speed" well below the easily anticipated operating speed and compensate by using above-minimum values for geometric design elements. This tug-of-war often produces reasonable results as long as you don't take the "design speed" to be anything other than a number on the cover sheet of the plans.
But the compromise can also fail. A highway with tangent sections and occasional 40 MPH curves does not have a 40 MPH design speed by any reasonable definition, but does in common American practice. The road is a trap, and pointing at the digits "40" on the highway plans or transferring those digits to speed limit signs does not make it less of a trap.
Krammes' paper recommends a method used in other countries where speed profiles are simulated and the design modified to lead to a more consistent speed by a feedback process. (Companion papers in TRR 1701 go into more detail on techniques to estimate speed reduction and accident rate at curves.)
Consistent with this advice, the Green Book should call for higher side friction values, higher design speeds, and more design consistency.
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