Tuesday, April 12, 2011

85 MPH: what's the point?

Last week, we learned that the Texas State Legislature is considering a bill that would raise the maximum speed limit on certain rural highways (generally in the western part of the state) from 80 to 85 mph.

This doesn't really make much sense to me. For starters, it seems kind of strange that something that reduces a vehicle's fuel economy is being suggested even as gasoline prices are continuing to rise.

But more importantly: raising the speed limit by five miles per hour on these highways doesn't really do much in terms of time savings, which is apparently the reason for this proposed change. This is because of diminishing returns: the faster you go, the smaller the difference, in terms of time savings, a relatively small 5 mph adjustment in the speed limit is going to make.

Currently, I-10 in West Texas has an 80 mph speed limit on a 432-mile stretch between milepost 62 (near the El Paso/Hudspeth County line) and milepost 494 (outside of Kerrville). If a motorist has the fuel capacity and the fatigue tolerance to drive this entire stretch without stopping, the trip currently takes them 5 hours and 24 minutes (432 miles / 80 mph = 5.4 hours) at the posted speed limit. Assuming this same stretch of roadway is upgraded to 85 mph in its entirely, the trip would be shortened to 5 hours and 5 minutes (432/85 = 5.08) - a time savings of only 19 minutes.

Of course, as we all know, motorists generally don't observe posted speed limits; rather, they drive five to ten miles per hour above them. So, a motorist traveling the entire distance between the El Paso County line and Kerrville at ten miles above the limit (90 mph) makes this trip in 4 hours and 48 minutes (432/90 = 4.8); if the speed limit is raised to 85 mph and they feel comfortable traveling (and manage to avoid being pulled over) at 95 mph, they can shorten their trip to 4 hours and 33 minutes (432/95 = 4.55). In other words, they save a whopping 15 minutes on this stretch of road.

And this only applies to daytime driving. Texas has a statewide nighttime speed limit of 65 mph, and I haven't heard anybody suggest that this limit be raised.

Given the relatively small time savings created by increasing the speed limit from 80 to 85, the next question in my mind is: why not go even higher? Why not a maximum speed limit of 90 or even 100 mph on these stretches of highway? Heck, why not eliminate speed limits entirely? If it works for Germany, why can't it work for Texas? (I say this, incidentally, as somebody who has traveled on the A6 in Bavaria at speeds approaching 120 mph. That was fun.)

Issues of fuel economy and driver safety (as we all learned in driver's ed, at speeds like these you're not "driving" a car so much as you are "aiming" it) aside, there are some big differences between Interstates and Autobahns. For one thing, Autobahns have different design standards than Interstates. More importantly, requirements for driving on the Autobahn (e.g. slower traffic always stays to the left and passing on the right is forbidden) are not as well-observed or as aggressively enforced in Texas as they are in Germany.

Besides, much of the Autobahn network does have maximum speed limits, and even on the stretches that don't there is a "recommended" maximum speed limit of 130 km/h. Which, incidentally, is just about the same speed as the current 80 mph limit on West Texas Interstates.

If this bill passes, engineering studies of the stretches of highway that are eligible for this change will be undertaken. The earliest these new limits would take effect is summer 2012.

Kuff and the Houston Press have more.


TexMike said...

I understand that the night time differential will be eliminated.

Thomas said...

You are correct, as I've noted in a new post.

M1EK said...

The 5-10 above is a myth; when limits were raised in TX the spread actually went down by a large margin.