Design speed shouldn’t block higher speed limits

Thanks to a bill recently signed by the governor, the Texas Transportation Commission, which oversees TxDOT, may establish speed limits up to 85 mph on any state highway.

The bill uses the word “designed”. I’m afraid this may be misinterpreted to mean the civil engineering concept of “design speed”.

“Design speed” is not the maximum safe speed. It’s only a tool to guide road design. At best, it’s a conservative first guess of a speed limit; it’s often OK to set higher speed limits.

The TTC still has to change some rules before we can possibly see higher limits. They’ll probably adopt whatever TxDOT recommends. I sent the below letter to TxDOT to encourage them to not conflate the bill’s language with the civil engineering concept. I got a positive response, but the proof will come when the TTC adopts the speed zoning procedure revisions.

The letter:

HB 1201, which was just signed by the governor, allows the TTC to set a speed limit up to 85 mph on any road provided that “that part of the highway system is designed to accommodate travel at that established speed or a higher speed” and a standard engineering study was run.

This is a good thing. The old 70 mph limit was legislated in 1963. It’s now 48 years later, and 85 mph is perfectly safe on many roads with our drastically improved vehicle and road technology.

Here’s my concern: I hope the word “designed” in the revised statute will not be misinterpreted and become a roadblock to higher speed limits.

There is a concept of “design speed” in civil engineering. However, a design speed is a poor guide for a road’s true maximum safe speed for at least three reasons:

1. A road’s design speed that of its worst part. Suppose a 50 mph road has a 40 mph curve. By definition, the road’s design speed is only 40 mph. In the real world, the road should be signed at 50 mph, and yellow warning diamonds would be posted at the curve recommending 40 mph.

2. Design speeds assume characteristics of vehicles and road technology of the past. So a design speed established in 2011 will assume the inferior stopping distances, power, and safety of vehicles from many years ago. Even worse, most Texas rural roads were designed decades ago (e.g., back when cars had poor drum brakes, biased ply tires, weak horsepower, little safety equipment, dim headlights, no ABS or stability control, etc.). Design speeds established way back then will certainly understate what today’s on-road fleet can safely handle.

3. MOST IMPORTANTLY: Per the AASHTO, the design speed is merely “a selected speed used to determine the various geometric design features of a roadway”. Therefore, it is really only a theoretical/laboratory measurement. Its purpose is not to determine a speed limit.

The design speed must not be interpreted as a maximum possible safe speed. At best, it is only a conservative “first guess” of an appropriate speed limit; a road’s true safe speed may easily be higher.

To conclude, I ask that, as the TTC revises speed zoning regulations to accommodate HB 1201, that it not hamstring the speed zoning process with the civil engineering concept of “design speed”. Certainly in its use of “designed”, the legislature did not mean to invoke this specific concept. In doing so, Texas would misuse a theoretical, laboratory measurement whose purpose was never to be an absolute cap on speed limits.

(AASHTO is the American Association of State Highway and Transportation Officials.)

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