Speed Limits - General Question

Until the day when we can re-engineer the driver, it's not insulting, it's reality.

Only about 1/3 of all crashes have an environmental factor (including road design) listed as a causitive factor on the police crash report. Many of these have no relation to engineering, like animal action or glare. Even fewer (~1/8) are vehicle related.

On the other hand, 95% of the reported causitive factors involve the driver. Our influence on these things are limited. No matter how well we engineer the road or the vehicle, the screaming kids in the back seat or the diabetic's plummeting glucose will take their toll. Not to mention DWS (driving while stupid).

As speeds increase, the driver's zone of useful vision narrows, and reaction speed certainly doesn't get faster. If anything, perception-reaction time may worsen because the distance at which things become important to the driver can exceed the eye's ability to resolve them. At the same time, the reaction time available to the driver decreases.

For example, say you are at a stop-controlled side road. A vehicle is approaching you at a high rate of speed. Intersection sight distance for 55 mph is 600 feet. At these distances, depth perception doesn't work. We use other visual cues like the rate of change of the apparent vehicle width (subtended angle) to judge speeds and distances. Because of this, you can't tell how fast a vehicle is approaching until it is about 600 feet away. So, you can't tell if a vehicle is coming at you faster than 55 mph, you can't tell how fast it is approaching until it is already too close to safely make the turn.

Also, the effects of an error are magnified. Injury severity is proportional to speed squared (as one might expect from Newtonian physics). The chances of a fatality actually increase with speed to the sixth power.

Lastly, it's incomplete to say that speed differential causes most of the crashes on freeways, since many if not most crashes occur during times of congestion, in which case the speed linit is prety much immaterial. Also, the 80 mph speed limit only applies to rural West TX speed limits, where congestion isn't a factor.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

I'd like to commend ACTrafficengr for an excellent post. A very well-considered, supported argument.

In most of the metric world, freeway speeds are 120 km/hr, which is about 75 mph; a speed limit of 80 mph is not, in that context, extreme.

ACTrafficengr's post *sounds* good, but then why did Montana's fatality rate increase then their speed limit went from unlimited to 75 mph? The reason Texas increased theirs was because there were fewer fatalities at 75 than at 70.

Sounds like priorities are not in keeping with reality.

According to Traffic Safety Facts 2005, there were 43 fatal rural interstate crashes in MT, and 213 in TX. With numbers that low, random variation in crashes could account for it. Two fatal crashes in MT is a 5% change in crash rate. For example, here are the overall fatality rates for MT (TSF 2005 doesn't list fatality rate by functional class):

Year Fatalities per 100 million veh miles traveled
1975 5.08
1985 3.03
1990 2.54
1995 2.28
2003 2.41

There is enough noise in there that a decrease could have been natural fluctuation. They could have also combined the speed limit increase with more enforcement an education.

There are also losts of counterexamples. Iowa, which has possibly the best crash database in the nation, saw a 500% increase in their fatality rate after raising speed limits from 55 to 65.

We could go on all day cherry-picking statistics, but the bare fact is, crash severity increases with kinetic energy, and, the higher the speeds when a crash does occur, the more likely it is someone will get hurt or killed.








"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

That's one bare fact. Another is that speed differential is the major cause of freeway accidents. With more people feeling safer driving faster (after billions of research, development, and vehicle and roadway improvements, most people do) it's safer to "go with the flow" rather than the dangerous dogma of "staying at the speed limit" and creating convolution.

What are the bare Iowa numbers, and why are their numbers better - or are they just higher, justifying your position?

Also, NHTSA is known for distorting information to justify their existence - The bare facts on that are that it's much safer to be driving on the interstate than to be out of your car. *That's* the bottom line.

I'm mot disagreeing with you about speed differential. It is as dangerous to go 10 mph slower than average than 10 mph faster.

I'm just trying to say that the big picture is more nuanced, and can't be simplified to "speed kills" or "speed differential is the culprit."

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

While traffic engineering is not my typical bag, I would like to say that I travel I-40 in AZ for about 20 miles on most days. The speed limit along this stretch is 75 mph.

I try to leave myself plenty of time so that I can go a few miles per hour slower in the name of better fuel economy since I know that nothing kills economy like wind resistance (and of course high accel. rates).

As speed limits are increased, this will by its very nature invite a greater discrepency in travel speeds and lend itself to more speed differential, thereby increasing risk.

Out of curiosity, what are statistics for the German Autobahn?

The German Autobahn is safer than US interstates. The safest speed limit is at the 85th percentile of the freeflow speed. Slower than that is just as dangerous as faster than that. Driving slowly means everybody behind you needs to watch out for you.

It is not a coincidence that the 85th percentile is one standard deviation above the mean.

I don't think we could sustain the design, maintenance and enforcement standards used on the Autobahn. For example, vertical curves are designed to an object height of 0, they have a zero-tolerance policy for pavement defects, and even obnoxious behavior is a ticketable offence, with the fine set based on your ability to pay. IIRC, a rude gesture is typically assessed a $600 fine (or was it DM 600?).

Although if Rudy Giuliani was still in office, NYCPD's quality of life campaign may have progressed to rude gestures by now.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

This thread has hit on one of my pet peaves.

If speed differential is a problem and I can get ticketed for driving faster than the speed limit (or signifacantly slower than the speed limit). Shouldn't the speed limit be set at a speed that is resonably safe for a majority of the people and more stictly enforced to remove the speed differential problem.

In my experience, many of the problems occur when functional class and land use don't jive. When the land adjacent to major collectors and minor arterials is zoned residential, people complain about the speed and volume of traffic on their "residential street."

This problem is exascerbated when a full-out AASHTO design with 12' lanes and 8 foot shoulders is used, and the speed limit is set at 30 mph because, after all, it is a "residential street."

Zone it commercial/industrial, or design it to foster lower
speeds, and the problems will be at least partially diminished, imho.

IHSDM has a speed model used to determine whether adjacent road segments are compatible (i.e., no 150' radius curves after a mile long tangent." It's calibarated for rural roads now, but the idea could be used in the future to make sure functional class, land use patterns and road design converge.


"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust