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The Insurance Institute for Highway Safety (IIHS) almost hyperventilates {link to their site} about 'design speed' while discussing speed zoning:
[Some proponent of higher speed limits, says the IIHS] claim that, because interstate highways meet rigid design standards for sight distance and roadway geometry, they should be posted at their design speeds [e.g. 70-mph on rural interstates]. The problem is that a design speed is not necessarily a safe travel speed. The American Association of State Highway and Transportation Officials defines design speed as "the maximum safe speed that can be maintained ...when conditions are so favorable that the design features govern." In other words, it is the maximum speed at which drivers can maintain a safe level of vehicle control on a particular section of highway under the conditions for which the highway was designed. Speed limits are set somewhat lower because conditions are not always favorable and design features do not always warrant higher speeds. Many motorists also assume there is a kind of built-in tolerance factor in speed limit enforcement, so they exceed the limit regardless of what it is.IIHS is wrong, as it so often is. [Gratuitous comment: IIHS' longwinded arguments, like the Ohio State Highway Patrol's, just cover up their unwillingness to declare any criteria for speed zoning. They're really arguing for "setting the speed limit as low as possible; you can't make too many drivers illegal (heh, heh)."]
It's obvious that highway "design speeds" are very conservative (in the engineering terms, they have a lot of "safety factors" built in). Otherwise, a 70-mph interstate speed limit would be self-enforcing... but clearly people don't automatically going flying off the interstate when doing 71-mph, 81-mph, or even 101-mph!
To understand the issue of design speed, we have to 'scratch beneath the surface' of traffic engineering. For public consumption, engineers bill design speed as the "maximum safe speed" for a highway segment, but the key question is: "safe" for what? In my opinion, "safe" means "safe so a hot pizza on the benchseat won't slide onto you", since design speed mainly refers to the design of curves (both horizontal and vertical, also known as bends and hills to most of us).
My opinion is based both on observing traffic engineers in action, and some material sent by one of the FHWA engineers working on a recent study of speed limits {link to FHWA website summarizing the study}. He referred me to a June 1988 article in Better Roads, entitled AASHTO Speed Guides Questioned. The article is by David R. Olivarez, PE; Traffic Engineer in the Arizona DOT. The articles notes [my bolding added]:
When designing roadways, the engineer makes every effort to meet the AASHTO [American Association of State Highway and Transportation Officials] criteria based on the established design speed. In many cases, it is not practical or feasible to satisfy the established criteria...Another item he sent me was one page from an untitled, undated article. With those caveats in mind, it is nonetheless fascinating reading [my bolding added]:AASTHO states that minimum stopping sight distances and minimum passing sight distances are direct functions of the design speed... [However,] the minimum passing site distance from the MUTCD [Manual of Uniform Traffic Control Devices] is about half the established design criteria. This is an example of AASHTO establishing one set of criteria for design and another for operations...
In the 26 years I've been active in traffic operation, I've never used the design speed direction in speed zoning work. This has never been a problem...
The fact is that a lone motorist driving a section of rural interstate at night with no other vehicle in sight can only see about 200 feet with high beams. If we were to set the speed limits based on AASHTO design criteria, our rural interstate would need a night speed limit of 30 mph. Realistically, one cannot use the design speed directly to establish speed limits...
If there is a flaw in the current speed zoning process, perhaps it is the almost automatic downward adjustments made to account for design, development, and accident history. Part of reason for downward adjustments in the speed zoning process is the perceived tie between the posted speed limit and the design speed of a highway segment.In Ohio, I have observed that ODOT had formerly design speed in their speed limit formula (replaced about 1992 in the OMUTCD). In that formula, it is important to note that design speed was used with a 10% 'weighting;' NOT as a maximum, not-to-exceed value. Furthermore, the 'new' OMUTCD formula omits design speed altogether. Curiously, ODOT reintroduced design speed as a factor in 1996 only for speed zoning multi-lane, divided highways with the new 65-mph limit. Again it was just one weighted factor, not the limiting value IIHS says it should be. I infer ODOT added it for zoning 65-mph and 60-mph roads because:The design speed of a highway segment is not the maximum safe travel speed under ideal conditions as commonly defined.
Instead, the design speed of a highway segment, in fact, refers to the minimum speed for which the road is designed. In other words, the most restrictive geometric features which make up a highway's design such as vertical and horizontal curvatures must safely allow an operating speed at least equal to the design speed, even under wet conditions with minimally acceptable tires. The values associated with a given design speed are by necessity very conservative in order to accommodate the many different vehicle types and mechanical capabilities, as well as wide range of driver abilities.
What the data indicate are that any reasonable driver, driving a vehicle of typical condition can safely exceed a section's design speed under almost any condition... If this were not the case, it seems that given the amount of daily vehicle travel, we would have far higher fatality numbers and rates. Perhaps the drivers whose vehicular speeds help determine the 85th percentile speed, are already accounting for all of these factors in their choice of problems.
Furthermore, when ODOT speed zoned State Route 835 in the City of Beavercreek, ODOT traffic engineers deliberately set the speed limit at 50-mph: 15-mph over the nominal 35-mph design speed! Again, the design speed sets minimum requirements for an entire highway segment: actual features may have much higher design speed characteristics. Along the miles of SR 835, only one half-mile S-section came near 35-mph in actual design (using a ballbank indicator, the City Engineer said). The City posted 35-mph advisory limits on the S-curve. I drive the S-curve at about 50-mph myself... unless I've foolishly left a hot pizza unbelted beside me.
Clearly, a straight, level highway has almost unlimited speed potential, since it has no curvatures that would be effected by 'design speed' criteria. You could leave a hot pizza beside you and roll along quite nicely.
And I suspect Mr. Olivarez's comment about "AASHTO establishing one set of criteria for design and another for operations" is fully on the mark. The City of Beavercreek had a 45-mph limit on North Fairfield Road between Dayton-Xenia and Col.Glenn highway, a carryover from when it was unincorporated Beavercreek township and ODOT's traffic engineers set the speed limit. However, then the City decided to widen the two-lane portion (between D-X and Pebble Creek) to match the already 4-lane portion northern segment. ODOT's design engineers forced the City to lower the speed limit to 35-mph to match the selected 35-mph design speed (well, when I questioned him on the matter, the City Engineer told me the ODOT design engineers threatened them with all sorts of liability problems if the City raised the limit back to 45-mph. As I recall, the CE said a 35-mph design speed was selected because a 45-mph design would have placed the sidewalks virtually on roadside residents' porches). So now you have this new multi-lane 35-mph area smoothly and directly connecting into an older 45-mph multi-lane area. Really stupid looking. So now you know the facts about the traffic engineering underworlds: there really is a difference between Operations folks and Design folks, i.e. ODOT can speak with forked tongue.
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June 06, 1997