HB 1353, which was effective Sept. 1, 2011, basically says any road can have a 75 MPH speed limit (full text) if justified by TxDOT’s speed zoning procedure. Before then, 75 mph was only allowed in a few, politically-chosen counties.
This speed zoning procedure generally requires the speed limit to be set at the 85th percentile speed of traffic.
What’s the 85th percentile speed? It’s the upper end of the “flow of traffic”.
Speed limits set at the 85th percentile speeds fully legalize this “flow of traffic”–the speeds of reasonable drivers–and only criminalizes demonstrably unsafe speeds. (more info)
What’s the 85th percentile speeds on NTTA roads?
On the Sam Rayburn Tollway (TX 121), the average 85th percentile speed across the entire roadway is 74.1 mph. That rounds to 75 mph. Virtually all of the tolled part of TX 121 could be set to 75 mph with no hassle.
- Between I-35E and I-635 has a 73 mph 85th percentile speed! That rounds to 75 mph! It’s debatable if the sag curves under many of the bridges really need a 50 mph advisory speed. If that could be dispensed with, or raised to, say, 60 mph, then you could easily see a 70 or 75 mph limit on the old part of the DNT!
- Between I-635 and the Collin County Line: 69 mph. This part could go to 70 mph.
- Between the Collin County line and Sam Rayburn (TX-121): 73 mph, another 75 mph candidate!
- Between 121 and the northern end: Surprisingly, only 69 mph! So the 70 mph limit there wouldn’t be changed. However, this measurement was taken when the speed limit was only 65. It’s 70 now, so the speed could have crept up a hair. This section needs to be rechecked.
If NTTA strictly followed the 85th percentile rule (which is only fair, since we motorists are expected to strictly obey traffic laws), and didn’t use arbitrary “engineering judgment” to force lower limits, we could see 75 mph limits on most of its road network.*
*A caveat: Because of a pointless, broken program called environmental speed limits, there may be red tape to raising the speed limit on DNT between Frankford Rd. and SRT (TX-121) and PGBT between I-35E and TX 78. None of the rest of the NTTA road system is subject to this program.
Here’s the raw data from NTTA: ORR-2-21-2012. This has the Excel files and strip maps. The Excel files have the speed checks. You’ll use the percentile function to get each location’s 85th percentile speed. The strip maps show where each speed check was run.