Anatomy of a speed limit

[James Young]

No, I don?t agree with that.  I strongly support advanced driver education and continuing skills improvement and believe that law enforcement has an opportunity to positively affect key measures of traffic safety by concentrating on the educational aspect of their function.

However, the empirical evidence gathered from all serious crashes and huge samples of traffic flow show that, even given the current population of drivers, key traffic safety measures have never been better. 

Traffic engineers have made the assumption ? after years of investigation ? that two key measures of traffic flow dominate the phenomenon:  the 85th percentile and the 10-mph pace.  The 85th percentile is the speed of the 85th percent slowest traffic; the pace is the ten-mph band that contains the most vehicles, both measured under free-flowing standards with minimum headway, i.e., adequate separation between vehicles so that one does not affect another.  Invariably, the 85th percentile will be within 1 mph of the top of the pace; IOW, if the pace is 64-73 mph, the 85th percentile is likely to be 73 mph and the posted limit should be 75 or 80 mph.  By empirical evidence, the cars least likely to be involved in a crash are those within the pace and the next 5% increment.  That is the very definition of safe ? absence of crashes.

Further, there is no reason to believe that lowering speed limits or draconian enforcement of extant speed limits will have any effect on the very real transgressions that you point out nor will it have any effect on the current number one cause of crashes:  impairment.  Politicians and police officials keep trying to solve a physical problem with the only weapon they have instead of collaborating with scientists and engineers to solve the problem.  Of course, when your only tool is a hammer, all your problems look like nails.  This has gone far beyond the purview and capability of any single institution to solve and using 19th century philosophy to solve 21st century issues is dumb.

[S204STi]

No, I don?t agree with that.  I strongly support advanced driver education and continuing skills improvement and believe that law enforcement has an opportunity to positively affect key measures of traffic safety by concentrating on the educational aspect of their function.

However, the empirical evidence gathered from all serious crashes and huge samples of traffic flow show that, even given the current population of drivers, key traffic safety measures have never been better. 

Traffic engineers have made the assumption ? after years of investigation ? that two key measures of traffic flow dominate the phenomenon:  the 85th percentile and the 10-mph pace.  The 85th percentile is the speed of the 85th percent slowest traffic; the pace is the ten-mph band that contains the most vehicles, both measured under free-flowing standards with minimum headway, i.e., adequate separation between vehicles so that one does not affect another.  Invariably, the 85th percentile will be within 1 mph of the top of the pace; IOW, if the pace is 64-73 mph, the 85th percentile is likely to be 73 mph and the posted limit should be 75 or 80 mph.  By empirical evidence, the cars least likely to be involved in a crash are those within the pace and the next 5% increment.  That is the very definition of safe ? absence of crashes.

Further, there is no reason to believe that lowering speed limits or draconian enforcement of extant speed limits will have any effect on the very real transgressions that you point out nor will it have any effect on the current number one cause of crashes:  impairment.  Politicians and police officials keep trying to solve a physical problem with the only weapon they have instead of collaborating with scientists and engineers to solve the problem.  Of course, when your only tool is a hammer, all your problems look like nails.  This has gone far beyond the purview and capability of any single institution to solve and using 19th century philosophy to solve 21st century issues is dumb.

James, first of all I disagree again with the notion that just because people drive at a certain speed that it is automatically safe.  That doesn't mean that I support a lower speed limit.  For example, if they lowered the speed limit around here to 65 or 55 you can bet your live that probably at least half of all drivers here will still be driving at 80-85mph if they get the chance.  Take a look at traffic in Phoenix, AZ sometime.  People drive at an unbelievably fast pace there.

Maybe I don't understand the 85% theory, but we have a broad mix of drivers who will actually drive well under the posted speed limit on the interstate (typically the elderly), and those who routinely drive over the speed limit.  During rush hour it's a constant surge/glide with traffic hitting 90+ to get around slower traffic and then having to slow as low as 55 to deal with some granny in the left lane, camping it out, next to a truck or something.

Basically my point is not to have draconian enforcement, but to say that because drivers are comfortable with a certain speed does not make it automatically safe.

[ChrisV]

A couple quick stats for you...

approx. 200 million registered drivers in the US.

Approx. 6 million accidents reported annually.

Approx. 3 million include injury.

Approx 40k are fatal.

What percentage of drivers are not in accidents annually?

What percentage of drivers are not injured annually?

If the average flow of traffic is higher than the speed limit, but the number of people getting into accidents is a small percentage, then it follows that the majority of people are driving safely, even at elevated speeds. This is why the 85th percentile rule works.

[James Young]

Maybe I expressed it unclearly.  It is not the fact that a majority of the people drive a particular speed that makes it safer than others but that they tend to migrate to their natural speed, a speed at which they feel comfortable and at which they believe serves their personal and economic needs.  All drivers do this calculus (related to risk homeostasis) internally, constantly and without reference to the posted limit.  Since most drivers are similar, they tend to have similar natural speeds where they individually are safer in their own minds and where, collectively, they actually are at lower risk of crashing. 

Imposing a speed limit well below the collective 85th percentile (of individual natural speeds) creates huge conflict because it tends to widen the 10-mph pace and subjects the safest drivers to unnecessary punishment, especially when the motivation to create conflict is revenue.  Phoenix is already on record as using their photo-enforcement to balance their budget.

You are quite correct in your observation that Phoenix drivers have a higher threshold of risk tolerance than, say, Chicago, because the driving environment in Phoenix is more conducive to higher speeds than Chicago, especially sightlines, which appear to play a major role in driving speed decisions.

Also, your observation that some drivers travel well below the posted limit is valid as is your observation that they cause disruption in traffic flow.  However, what we have discovered when limits are posted correctly, these drivers tend to speed up into the 10-mph pace, causing less disruption and fewer chances for conflict; we have also discovered the band between the 85th and 95th percentile (which may be only 3 mph wide) tend to remain static or slow down because they perceive the limit as legitimate, whereas previously they perceived it as bogus.

There is no 85th percentile concept in rush hour traffic; any attempted measurement would be meaningless.

We certainly agree that draconian enforcement is worthless or damaging.

[S204STi]

I was observing traffic today during a drive to Denver and back, and I have to say that you have some valid points.  Thanks for your explanation.  It makes sense to keep drivers in generally the same speed bracket, to avoid the huge disparity in speed which can cause accidents.

[Eye of the Tiger]

Just thought I'd post another article about how speed limits are just government money making scams:

http://www.thenewspaper.com/news/20/2085.asp

Australia: New Speed Limits Costs Money, Lives
Imposing a speed limit on the Northern Territories, Australia has increased the number of road deaths while destroying local tourism.

NT Speed LimitThe imposition of a speed limit on the formerly limit-free open highways in the Northern Territories, Australia is destroying local business without providing a corresponding safety benefit. In January, the top legal speed in the Territories was lowered to 130km/h (80 MPH) in an effort to generate revenue with a new traffic police force collecting newly doubled fines.

Before, automakers from around the world jumped at the ability to travel to the city of Alice Springs to test vehicles at their maximum speed in a warm climate. With the new speed limit, however, the testing business has moved to other countries. Imposition of the speed limit will likely cost the city up to A$9 million a year in lost hotel and tourism revenue.

The Country Liberal Party cited statistics that show the new limit has failed to improve road safety. The total number of highway deaths so far this year with the speed limit has already exceeded the number of deaths in 2006 when speeds were unlimited.

"There was no evidence that the open road speed limit was a significant factor in the Territory's high road toll," Shadow Minister for Transport Fay Miller said in a statement. "As so many people predicted at the time, imposing an open road speed limit has failed to cut the road toll. The open road speed limit was just another pointless attack by the Martin Labor Government on the Territory's lifestyle."

"Government have lied to us as the real problem is on dirt roads and not on the sealed open road," the Keep NT Limit Free website explained.

Source: Govt defends speed limits, despite road and economic toll (Australian Broadcasting Corporation, 11/20/2007)

[Eye of the Tiger]

In other news, I was about three seconds away from death today. A white Saturn SL2 came flying over the crest of a hill on a residential street at about 90-100 mph, right before I was about to make a left turn. If I had been there three seconds sooner, I'd be dead.

[Raza]

One of the roads in my area recently had its speed limit raised from 45 to 55.  As a 45, the average speed of cars was 60-65mph.  Now it's closer to 50. 

Most 35mph roads have average speeds of 50 in my area.  Most 45s average 40.  It's weird.

[James Young]

One of the roads in my area recently had its speed limit raised from 45 to 55.  As a 45, the average speed of cars was 60-65mph.  Now it's closer to 50.  Most 35mph roads have average speeds of 50 in my area.  Most 45s average 40.  It's weird.

No, it is a fairly common phenomenon.  Those drivers who religiously follow the limit without regard to its rationality tend to speed up when the limit is increased.  The weird part is that rational drivers who see the limit raised tend to treat it as more realistic and will slow down a little.  Empirical evidence from the Michigan State Police and Colorado State Patrol reflect the same phenomenon.

[hounddog]

I strongly support advanced driver education and continuing skills improvement and believe that law enforcement has an opportunity to positively affect key measures of traffic safety by concentrating on the educational aspect of their function.

As I am certain to regret this, and have asked you in the past regarding this, and having never received an answer, I will ask once again;

Who is going to pay for LE "concentrating on the educational aspect of their function?"  Where is this money supposed to come from?

[hounddog]

Empirical evidence from the Michigan State Police .... reflect the same phenomenon.

Regarding ONLY highway and freeway speeds.  MSP does not conduct such research on secondary roads as those roads speeds are now controlled by local government (villages, cities, townships and counties) by statute.  It would be pointless, meaningless and financially fruitless for them to conduct such study on roads where they do not control how, or why, speeds are set.

[James Young]

As I am certain to regret this, and have asked you in the past regarding this, and having never received an answer, I will ask once again;

Who is going to pay for LE "concentrating on the educational aspect of their function?"  Where is this money supposed to come from?

One need never regret a question but only the questions never asked.  Who pays for the law enforcement function now?  The taxpayer.  Shifting the emphasis from one aspect to a more beneficial aspect should require no more funds than are currently available and I would expect a better return for every marginal dollar spent.

We also have drivers paying much of their own freight and that would continue.  I paid for my own Bondurant School as well as for both of my now-adult children and consider it money extremely well spent. 

If additional funds are required ? not just desired ? the current beneficiaries of law enforcement are going to have to put their money where their greedy mouths are.  I speak, of course, of the insurance industry that has been ripping off the public for 60 years.  If they want real traffic safety, they need to help pay for it, not just donate a lidar unit to the Dallas PD, from which they make many thousands.

[hounddog]

Obviously, police are paid for by tax-payers whos own president believes need to be paying more of their wages to the government to frivilously waste.  Lots more. 

The lions share of traffic enforcement comes from regular general patrols in this country.  General patrols being that which is there to answer calls for service and enforce laws while on regular patrol.  How do you envision telling local governments they they are now taking money from the general patrols and using it for public training?  Right now many municipalities are laying people off, some in the hundreds and many in the dozens.  No one has the money to staff regular patrols right now, how would they populate and pay for a training staff to teach people what they should have already gotten from their early training, "going fast has certain inherent dangers.  Crashes sometimes kill.  Many of you will be in at least one crash.  You have to do your best to obey the traffic laws."

Is your answer to take even more money out of the already struggling, and heavily tax burdened American public by once again raising taxes on a plan that will not work any better than what we are currently doing, which by the way is what the majority of people ask for; more patrols, not less?

Insurance companies will never ante up.  They will spend billions fighting something whereby they are required to pay into the system, and you know it.  In the end, once again, it will be average Joe Schmuckatelli that would be stuck footing the bill on such an unrealistic idea as "education will solve everything."

My last question is; If you are underfunding patrols to fund training what good does it do if there is no one to keep track of those who still refuse to follow the rules?

[GoCougs]

LE is not about education, it's about, well, enforcement.

The leftist mindset of "education" and its antecedent, rehabilitation, is a chief state consolidator; it also plainly does not work.

The touchy-feelie thought may make some feel good, but only consequences drive human action.

[James Young]

hounddog writes:  {Obviously, police are paid for by tax-payers whos own president believes need to be paying more of their wages to the government to frivilously waste. }

Police services are paid for by states and municipalities;  policy directives are formulated at the state or local level.  The president and federal income taxes are irrelevant in this matter.

{The lions share of traffic enforcement comes from regular general patrols in this country.  General patrols being that which is there to answer calls for service and enforce laws while on regular patrol.}

This may be true for your region; it is generally true in Los Angeles and the San Francisco Bay region  as well.  However, throughout much of the South, Texas and especially in Oklahoma, village cops ARE traffic  patrol and do nothing other than traffic, leaving those duties to legitimate enforcement such as country sheriffs.  Further, cities in the region dedicate entire divisions to nothing but traffic patrol.  This is certainly true of Austin, Tulsa, Dallas-Ft. Worth metroplex, Denver-metro and probably others as well.

{ How do you envision telling local governments they they are now taking money from the general patrols and using it for public training?}

Those are your words, not mine, nor do they reflect what I wrote with respect to the education function of law enforcement.  Nobody has suggested that police forces add staff dedicated solely to ?education.?  That is but a red herring.

{Is your answer to take even more money out of the already struggling, and heavily tax burdened American public by once again raising taxes on a plan that will not work any better than what we are currently doing, which by the way is what the majority of people ask for; more patrols, not less?}

No, that is not at all what I am suggesting for, once again, those are your words, not mine.  Further your assumption that the majority of people are asking for more patrols is unsupported by evidence.  When elections are held for increased enforcement, usually in the form of public authorization or condemnation of red light camera and photo-radar enforcement or tax assessments for enforcement expansion, the public has rejected increased enforcement in every single case.  At best, the people are asking for effective patrols, not more patrols.  We have had many in these fora who continue to mistake visibility and effort for effectiveness. 

{Insurance companies will never ante up.  They will spend billions fighting something whereby they are required to pay into the system, and you know it.  In the end, once again, it will be average Joe Schmuckatelli that would be stuck footing the bill on such an unrealistic idea as "education will solve everything."}

Are you suggesting that we should ignore the insurance industry just because they are stubborn and greedy?  They have been sucking hundreds of billions out of the system for years and their operating policies are predictable only if one uses revenue generation (surcharges) as an assumption instead of decreases in crash-, injury- and fatality rates.  And, again, nobody is suggesting that ?education will solve everything.?  Those are your words.  Education can have an effect and all of us can benefit from additional knowledge, techniques of learning, but most importantly to critically question conventional wisdom.  I am 63 years old and just finished my doctorate, so I am a firm believer in education and critical thinking.  I short, I have put my money and my effort where my mouth is and I am willing to question my own views, something sadly missing from so many people whose views are fixed and necessitate a molding of facts to the view, not vice versa.  One of the most difficult lessons that I learned was to let go of certain beliefs that had no basis in reality.  I expect nothing of others that I do not do myself. 

[hotrodalex]

In other news, I was about three seconds away from death today. A white Saturn SL2 came flying over the crest of a hill on a residential street at about 90-100 mph, right before I was about to make a left turn. If I had been there three seconds sooner, I'd be dead.

That was me. I figured I should test the top speed of my brother's car. 

[NomisR]

That was me. I figured I should test the top speed of my brother's car. 

You missed!

[hotrodalex]

You missed!

I know. It was faster than I thought.