Take a Closer Look—Part 2

By John Carr, NMA Massachusetts Activist

 “Pay no attention to that man behind the curtain.” — The Wizard of Oz

Last time I gave some examples where engineers didn’t even bother to make excuses. They wrote a deficient report and hoped nobody would notice. Often you’ll get a report that tries to explain.

Here’s something important to remember for most of the controversial topics: Most of the time they don’t know any more than you do.

There are experts on the subject. The guy making excuses isn’t one of them.

The average city traffic engineer is experienced in turning a traffic complaint into a resolution. He knows how to measure roads and traffic. He is not an expert on the theory of speed limits or stop signs. He is listening to feedback from the city council instead of doing statistical analyses to find out whether the new speed limit improved safety.

There have been methodical, controlled, before-and-after studies of speed limit and stop sign rules. The “Parker study” found that posting lower numbers did not make roads safer. Other studies found that posting four way stops at low volume intersections was harmful.

That is why I’m not afraid to say I’m right and the guy with 20 years experience giving politicians what they want is wrong.

Let’s look at some speed limit and stop sign excuses that don’t pass close inspection. As a reminder, in typical circumstances speed limits should be close to the 85th percentile speed of free-flowing traffic. MUTCD 2B.13. (Some say in all circumstances. Let’s say usually for now.) Four way stop signs are meant for busy intersections where traffic backs up or there are a lot of accidents. MUTCD 2B.07.

We’re all special

Watch out for exceptions that swallow the rule. Here’s an obvious one.

An intersection in Worcester, Massachusetts had no accident problem and a tiny fraction of the minimum volume required before four way stop signs should be used. There was a political demand for stop signs to slow traffic. Stop signs aren’t supposed to be posted at low volume intersections or for speed control. The DPW came back with a report explaining why stop sign standards could be ignored:

However, due to the nature of this neighborhood, which is 100% residential, the stop sign installations may resolve the majority of the speeding and may reduce some of the cut-through traffic.

The “residential” excuse is obviously illegitimate. Residential areas are neither unusual nor special. Urban and rural streets have used the same sign books since 1935, the Manual on Uniform Traffic Control Devices. (Texas A&M describes the history of the MUTCD.) Ignoring the rule in residential areas is just plain ignoring the rule.

The city solicitor advised the city clerk not to sign the ordinance. The signs were posted anyway. They aren’t legal, but they are posted and that’s what the city cares about.

Unbounded deviations

If there’s a factor that may justify a change, how much change does it really justify? In speed zoning 5 mph is a lot. Usually most traffic is within a 10 mph range (called the “pace”). Reducing speed by 5 mph below the 85th percentile speed raises the violation rate to 50%. By 10 mph, 85%. By 20 mph, typical on Interstates in the Northeast, 95-99%.

A speed limit reduction to 10 mph or more below the 85th percentile speed is the same as ignoring the 85th percentile speed.

Speed zoning standards that quantify acceptable deviation from 85th percentile speed usually limit it to 5-8 miles per hour. For example federal guidelines say 5 mph, California says normally 2 mph but up to 7 mph (round to nearest 5 mph and maybe subtract 5), and Florida says normally 3 mph but up to 8 mph.

If the speed limit could have been posted 60 but was posted 55 due to a high accident rate, that is consistent with standards. (Not necessarily a good idea, but that discussion can wait.) What happens in real life is the speed limit starts at 50, somebody photogenic or popular dies in a wreck, and the limit is lowered to 35. Accidents may justify a 5 mph reduction. They never justify a 15 mph reduction.

Arbitrary numbers

In 2003 MassHighway set a speed limit on the newly rebuilt Route 62 in the town of Princeton. In one area the 85th percentile speed was 53 mph, there were no curves, there were no speed-related accidents, and the area was described as “very sparse residential.” Trial runs showed 55 to be reasonable. So all the factors pointed to a 55 mph speed limit.

The speed limit there is now 45. State records show how the report turned into a speed limit. Somebody wrote a note on the page: “UNDIVIDED 50 is proper!” So 55 became 50. A selectman said he was “appalled” by the prospect of a 50 mph speed limit. (The police chief didn’t have a problem with it.) Another handwritten note, and 50 became 45.

The notes could just as well have said “RESIDENTIAL 30 is proper!” and “town manager prefers 25.” All these numbers are just somebody’s opinion. Use of unsupported opinions are exactly what the engineering study process and 85th percentile rule are supposed to prevent.

What happened is somebody decided that two lane roads should not have speed limits over 50 mph. Somebody else decided that his town should not have any speed limits beginning with “5”. With two arbitrary 5 mph reductions the state turned a proper speed limit into one most drivers ignore.

The speed limit approaching a left exit must be lowered to 45 (old rule in Massachusetts). The speed limit within city limits can’t be over 35 (common in Nevada).

When you see an alteration like this ask yourself: Why 50? Why 45? Why 35? Nothing in any accepted speed zoning standards says 55 is too fast for a two lane road or politicians get 5 mph for the asking.

Miles away from here

In some states the DOT will reject a speed limit increase because the resulting speed zone would be too short. “Too short” can mean less than ten miles long.

You will find this minimum distance is not consistently applied. In Massachusetts a proposed three mile long speed limit increase was described as too short. There are countless places around the state where the speed limit increases or decreases for less than half a mile. Why is it OK to go 45-50-45 in one mile but not 50-55-50 in two miles? Why can a 55 mph be three miles long but the same section is too short for 60? Obviously this is not a real engineering standard, but rather an excuse.

In the 1980s Congress relaxed the national speed limit to allow rural Interstates to be posted 65. Somebody decided this was too much freedom and created a guideline that 65 zones should be special, not routine, and should not be posted unless the speed zone would be at least 10 miles long. This old rule is still cited.

There is no basis for the 10 mile limit. It does not take five miles to accelerate to 65 and five more miles to brake back down to 55. It takes 100 feet if you use your brakes and a quarter mile if you just let off the gas. If the speed limit can be 55 for half a mile it can be 65 for one mile.

Ask yourself: Why one mile, three miles, or ten miles? Is the policy consistently applied, or cited only to prevent possibly controversial speed limit increases?


I gave some examples where the official waved a magic wand. It’s residential, it’s undivided, it’s only a mile long. When you hear that, ask yourself: What if that excuse were used every time it was applicable? That means every intersection in a residential area has a four way stop, every street with a house gets whatever low speed limit is being pushed for residential areas, every speed zone less than ten miles long is reduced to match the surrounding limits. Would the result be absurd? Would it eliminate the purpose behind the underlying rule? Not a very good excuse then.

One final note. I’m not calling these people idiots. Like I keep saying, it’s public relations. Many DPW and DOT engineers understand professional standards, know that their excuses are lies, and choose to keep their jobs rather than be fired for doing the right thing. That is part of the deal for government employees.

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