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Why are there no speed limits posted on the new 183 South toll road?

Blurred traffic on a highway
Gabriel C. Pérez
The new 183 South toll road doesn't have any speed limit signs.

The new 183 South toll road has been open since early 2021. The 8-mile highway runs from U.S. 290 East down to Texas 71 near the airport.

While driving down 183 South, a KUT listener noticed something was missing.

"I'm wanting to know what the speed limit is on the tollway," Carolyn Estes asked KUT's ATXplained project. "It's not posted anywhere that I have been able to find, and I would like to avoid getting a speeding ticket."

She's right. More than a year after opening, 183 South has no posted speed limits. But that doesn't mean you can drive as fast as you want.

In Texas, most highways have a default speed limit of 70 miles an hour unless there's some hazard that requires you drive slower.

Of course, speed limits don't have to be 70 miles an hour. For the past year, the Central Texas Regional Mobility Authority has been doing a study to figure out what the posted speed limit should be on 183 South.

"We basically hire a firm. They go out there during off-peak periods when traffic is moving freely. They set out devices that collect the speeds of vehicles. Collect all that data, run it through an analysis," Mike Sexton, the CTRMA's acting director of engineering, said.

The analysis determines something called the 85th percentile speed. That's the speed at which 85% of cars are traveling at or below.

Put another way, if you had 100 cars driving down the road, the 15th fastest car would be the speed they're seeking.

"That's determined to be the most safe and prudent speed for the corridor," Sexton said. "That's the speed that you drive feeling safe on the roadways, and generally the rest of the vehicles around you are kind of doing that same speed."

The method is outlined in a TxDOT manual and is used for highways all over the state.

But some observers say the 85th percentile speed is not the best way to determine a speed limit, even though the system has been used for decades based on research dating back to the 1940s.

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"That research has since sort of been, I don't know if debunked is the right word, but that research has since sort of been reevaluated," said Jenny O'Connell, a senior program associate with the National Association of City Transportation Officials, a nonprofit group focused on urban mobility.

"We know that drivers are tethering their speed to a number of different things, but one of those factors is certainly the speed limit. So when the speed limit rises, speeds also rise," O'Connell said.

"Conversely, when the speed limits drop, speeds also drop. And those speeds especially drop at the very high end, so 10 or 15 miles per hour plus over the speed limit, and those are the highest risk speeds," she said.

Lower speed limits can make the worst speeders go slower, and speed is the most common factor in traffic deaths.

But in Texas, if the speed limit is too low for how a road was designed, people might not have to obey it.

Under state law, if you're over the speed limit, you are presumed guilty of speeding. But if you can prove the speed you were going was appropriate and reasonable, technically, you weren’t breaking the law.

In other words, Texas law doesn't enforce a numerical speed limit. The statute just requires that you drive at a speed considered to be "reasonable and prudent under the circumstances." Texas is among a number of states to have adopted the standard after Congress in 1995 repealed a national speed limit of 55 miles per hour.

As for 183 South, the yearlong speed limit study is done. CTRMA would not reveal what speeds the study recommended. But we'll find out later this month.

The CTRMA's board of directors is scheduled to vote April 25 on the recommended speed limit. After that, speed limit signs can go up, and Jenny Estes and anyone else driving down 183 South will at least know how fast they can drive without getting a ticket.

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Nathan Bernier is the transportation reporter at KUT. He covers the big projects that are reshaping how we get around Austin, like the I-35 overhaul, the airport's rapid growth and the multibillion-dollar transit expansion Project Connect. He also focuses on the daily changes that affect how we walk, bike and drive around the city. Got a tip? Email him at Follow him on X @KUTnathan.
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