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5:39 am EDT        63°F (17°C) in Iowa City, IA

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Yes, I know I'm nuts for being awake at this ungodly hour, but I need to do something to stay awake long enough to deliver my load here. Why not make yet another update? wink

This time, I'm going to talk about some changes a few states have made recently which will affect me on the highways. Three states, including the one I am currently in, have just passed legislation raising the statutory speed limit on at least some of their Interstate freeways. As of this past Friday, the speed limit on rural Interstates here in Iowa has been raised from 65 mph to 70 mph for all traffic; the speed limits remain at 65 near the Quad Cities, on I-80 near Iowa City, along I-35/80 in Des Moines, and on I-35 near Ames (and at 55 mph on I-235 in downtown Des Moines and on I-80 and I-29 in Council Bluffs).

Today, a raised speed limit takes effect two states to my east in Indiana. All sections of Interstate or compatible freeway that had been eligible for the statutory maximum of 65 mph for cars and 60 mph for trucks will have both of those raised by five. Hoosier motorists will be able to drive 70 mph in cars and 65 mph in trucks.

Finally, Texas legislators voted in late June to permit an 80 mph daytime speed limit for cars on sections of Interstate in very sparsely populated (less than 10 persons per square mile) counties. For the most part, this only applies to parts of I-10 and I-20 in far western parts of the Lone Star State. Additionally, the population-density threshold for allowing 75 mph daytime car speed limits on Interstates has been increased. That threshold used to be the aforementioned 10 persons per square mile; now, counties with between 10 and 15 persons per square mile qualify for 75 mph. Statutory daytime truck speed limits (and daytime car speed limits in counties with more than 15 persons per square mile) remain at 70 mph statewide, and the statutory nighttime speed limit of 65 mph for all traffic also remains in effect throughout Texas.

I can't say too much for other states, but I think speed limit policy in my home state of Michigan could use a few tweaks. First, the nation's widest and most dangerous car-truck split limit (70 mph for cars, 55 mph for trucks) must be reduced or eliminated; seeing as most trucks are going (and getting away with, due to lack of enforcement) 65 mph, the truck speed limit should be set at 65 on freeways. In the southern Lower Peninsula, I would extend 65-mph "zoned" speed limit segments farther out from Detroit, and institute 65-mph zones in high-traffic semi-rural areas near the major cities of Ann Arbor, Grand Rapids, Lansing, Flint, and Saginaw. Up north and in da U.P., traffic is much lighter and distances between population concentrations are longer; I would not only raise the speed limit on freeways to 75 mph for all traffic, but also raise the speed limit on state-maintained two-lane roads to 65 mph. Everybody is already going 65 up north even with the current 55 mph limit on two-lane roads, so it would make sense to set the speed limit to 65 on these highways.

A frequent discussion on the USENET newsgroup misc.transport.road (much of which actually spills over from involves the theory of proper speed limits. Some m.t.r. and r.a.d. posters are members of the National Motorists Association (NMA), which proposes doing traffic-speed studies in the absence of enforcement and then setting the speed limit to the 85th percentile speed of traffic (that is, a speed which 85% of traffic travels at or below, and 15% of traffic exceeds). I'm not convinced it's quite that simple, based on 3½ years out here in a truck with 315,000 miles (more than most people drive in 30 years) behind me, and I propose a different theory.

In the absence (or assuming ignorance) of speed limits, my experience would suggest that it is the trucks on a freeway that set the tone for speed. Yes, there are a few bad apples who are so dangerous they ought not have a CDL, and there are a lot of companies that govern their trucks to 65 mph or less, but for the most part, truck drivers will select a speed that they feel is safe for conditions of weather, daylight, traffic, and road surface. In draconian-enforcement 55-mph truck limit states like Ohio, the average truck is likely traveling around 63 mph in free-flow conditions; in higher-limit states such as Nebraska (75 mph for all traffic), the average truck probably isn't even hitting that, going maybe 72 mph or so. In general, the great majority of car drivers (ignoring the few out-liers just screaming down the road) seem to select a speed 3-7 mph faster than the average truck.

My suggestion is to take that average speed of trucks in rural free-flow conditions, and set the speed limit for all traffic to the next highest 5-mph increment. For example, in Nebraska, this would work exactly as the Legislature has set it: based on an average free-flow truck speed of 72 mph, the speed limit would be set to 75. In Ohio, I might suggest a little leeway from the letter of my idea; while trucks should have a 65 mph speed limit, based on my theory, I wouldn't have a problem with maintaining the split speed limit and allowing cars to go 70 mph.

Expanding on the split speed limit topic a bit, I offer the following corollary to my speed limit theory: split car-truck limits, where they exist, must be no more than 5 mph apart. Any greater split, such as the current 65/55 in Illinois, Ohio, and Oregon, 70/55 in Michigan and California, 70/60 in Washington state, and 75/65 in Montana, creates unnecessary "road rage" among all types of traffic; car drivers get annoyed with being held up while one much-slower truck passes another much-slower truck, and truck drivers get annoyed at being unable to pass any slower vehicle (because the much faster cars, especially in a long nose-to-tail line, won't let them change lanes). Assuming a reasonable level of speed enforcement (which isn't the case in Michigan — they're almost never out there), the greater the car-truck split, the worse this artificially-created road rage effect becomes.