Posted speed limits on Texas tollways going to 80mph - America's fastest pikes
By Peter Samuel
Posted speed limits on 54miles, 87km of tollroad in the Austin Texas area are being increased to 80mph, 129km/hr from their present 75mph, 121km/hr. The 42 mile long TX130 along the east side of the Texan capital metro area and 8 miles of TX45-SE on the southern periphery now make 80mph driving legal.
The change was formally implemented with a decision at the latest meeting of the Texas Transportation Commission on a recommendation by Texas Department of Transportation (TxDOT) which runs the two tollroads.
Speed limits of up to 85mph, 137km/hr are allowable under Texas law.
The two speedy tollroads go from I-35 near Georgetown north of Austin and rejoin I-35 at Buda south of the state capital, forming something of a bypass for long-distance traffic between Dallas and San Antonio. The 'interstate' going through the middle of Austin and often ten lanes (2x3 mainline lanes and 2x2 frontage road lanes) is overwhelmingly devoted to internal Austin area traffic and has clear peak hour congestion associated with commuter travel.
I-35 55mp and 65mph through Austin
I-35 north of TX130 toward Dallas is signed for 75mph, but through the Austin area itself is signed for 65mph, and in places 55mph. That's a 12 lane highway with 2x3 frontage road lanes, and 2x3 mainline lanes. It has major interchanges and quite close-spaced slip lanes between the mainlanes and the frontage road lanes, and considerable weaving movement amid often dense traffic.
TX130 and 45SE by contrast are just 2x2 lanes plus shoulders with a wide 100ft grass median in the middle. Only in places do they have frontage roads and associated slip lanes.
In a statement Carol Rawson, TxDOT Traffic Operations director is quoted: "These higher speed limits on SH 130 and SH 45 Southeast provide travelers a safe and efficient alternative to the congestion on I-35 in Austin. Though the distance is longer, the actual travel time is comparable and more reliable." (SH is Texan usage for state highway, the equivalent of SR for state route or state road in other states. Our usage is the two character state abbreviation, in this case TX.)
As compared to the 54 miles of TX130/45SE bypass of the Austin area, the interstate I-35 is 43 miles, making the tollroads 11 miles or 25% longer because of the jog east.
By our calculation when trips follow the new posted speeds and average 60mph they take 43 minutes on I-35 and 41 minutes on the tollroads - about the same travel time. Incidents are more common on I-35 however so the toll bypass TX130/45SE probably has a reliability advantage over the more direct interstate.
In peak periods in Austin the tollroads should have a time advantage. If average speed on I-35 drops to 45mph because of congestion the same trip takes 57 minutes. And if the tollroads remain free flowing at 80mph there is a 16 minute advantage (57 - 41) over the Interstate.
Other Austin area tollroads
TX45 North an east-west TxDOT tollroad with denser traffic than TX130 and TX45SE got 75mph signs in place of 70mph in a decision October 2011.
TX183A a tollroad operated by North Central Texas RMA in the northwest suburbs of Austin has a 70mph, 113km/hr posted speed limit, and an official there said they plan to study this some time after they open their second stage. That's happening any day. After the longer TX183A has been in use they'll be studying speeds and considering whether they need to adjust them. (CORRECTION)
The shorter Mopac Loop 1 (not in fact a loop but a north-south highway) has a tollroad section among the busiest pikes of the area and 10 lanes (4 frontage, 6 mainlanes.) It is posted 65mph.
85th percentile guide
Officials said today that the practice in Texas is to use as the major guide the "85th percentile" - the speed that sets 85% of drivers below and 15% of drivers above by measurements of actual traffic speeds in normal conditions and good weather.
Posted speeds are usually set to the nearest 5mph to have, say, either 65mph or 70mph posted. Lower numbers than the 85th percentile are sometimes used if the highway is sub-standard with narrow shoulders, if there is construction work, or a bad crash record.
A TxDOT statement says: "The posted limit may be lowered (by as much as) 10 mph below the 85th percentile speed if some of the following conditions are present:
- Roadway pavement width of 20 feet or less
- Curves and hills
- Hidden driveways and other developments
- High number of driveways
- Crash history
- Rural residential or developed area
- Lack of striped, improved shoulders"
Not much of this applies to expressway standard highways.
In a comment on setting posted speeds too low TxDOT says:
"If reasonable drivers see an unreasonably low speed limit without seeing a need to drive that slowly, they tend to ignore the signs and develop disrespect for speed limits in general.
"When a speed limit is set below the 85th percentile, law enforcement officials must deal with reasonable people being ticketed for exceeding the posted limit as well as motorists who drive too fast."
Overall Texas now has more than 3,000 miles, 5,000km zoned at 75 mph or higher, and 575 miles, 920km posted at 80 mph.
Maryland inverts the 85th percentile rule for its new tollroad
The joke here in Maryland is that the state DOT got confused in setting the posted speed at 55mph for the new Inter County Connector, MD200 tollroad in the northern suburbs of Washington DC area.
They set 55mph as the "85th percentile" on the basis, the joke goes, that 85% of drivers on the ICC exceed the posted speed and 15% are below it.