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The Effect Of Speed Limits On Actual Travel Speeds

Posted on August 21st, 2008 in | 1,549 Comments

The Effect Of Speed Limits On Actual Travel Speeds
By Jim Walker, NMA Michigan Member

I have worked closely with the Michigan State Police for several years in their pursuit of correcting as many Michigan posted speed limits to the correct 85th percentile speed level as possible. Yes, we have a very enlightened state police administration that wants to see posted limits set for safety, not revenue.

I have testified before Michigan legislative committees in support of the State Police to help explain the science involved, helped to nominate the key officers for a Governor’s Traffic Safety Advisory Committee Award which they won in 2006, and helped the police find areas of state trunk line routes (numbered highways) which should be re-surveyed because the posted limits were set far below the normal speeds of traffic.

In late 2006, the state police came to Ann Arbor and did speed studies on several state routes through Ann Arbor, parts of Business Route US-23 and parts of Business I-94.  The posted limits on these trunk line routes are legally under the control of the state police and MDOT, not local authorities, but the local authorities can sometimes “push back” in the court of public opinion.

After a long period of negotiations and explanations with a city that does not want posted limits raised at all, three areas were re-posted in early 2008 with corrected speed limits raised to the 85th percentile speed of free flowing traffic under good conditions.

The City Council even passed a resolution opposing these safety-oriented changes, but they do not have legal control over state routes, so they finally agreed to the three areas to be changed.

After allowing a period of adjustment while drivers got used to the newly posted higher limits, I re-surveyed these three areas to see what changes there were, if any, in actual travel speeds.

The huge study done in 1992 by Martin Parker says there would be little change in the speeds people actually drive.

This was, of course, the result.

Actual travel speeds changed by a maximum of 2 mph in some parameters, not at all in others, and some speed points were lower with the higher posted limits. The actual traffic speeds remained the same as they have been for 23 years.

One thing did change. As was expected, the vast majority of safe, sane, competent drivers who go along with the normal flow of traffic are no longer arbitrarily defined as criminals, and no longer subject to big ticket fines and even bigger insurance surcharges.

One of my key goals is to get a reluctant Ann Arbor city government to adopt the proven practices to set the safest speed limits as described in the Institute of Transportation Engineers Engineering Handbook, the Michigan Manual of Uniform Traffic Control Devices, and the revised set of Michigan traffic laws that went into effect in November of 2006.

It is an uphill battle, because of two reasons.

First, the city makes so much money from traffic tickets that safety practices take a back seat to the revenue.

Second, the flow of misinformation and deliberate disinformation that has come out of Washington since the early 1970s has convinced many citizens that lower numbers painted on the speed limit signs means lower actual traffic speeds and safer driving.

Anyone who has read the scientific literature knows this is totally false, but a lot of education is needed to repair the damage and correct the false beliefs many people have about posted limits.

Hopefully the City Council members and others who read the charts will see the proofs that actual travel speeds do NOT rise with corrected 85th percentile posted speed limits and that will remove one counter argument for posting 85th percentile speed limits to maximize safety.

RESULTS
Definitions included at the bottom of the page.

History Of Speeds On North Main Street (Northern Section)
Data is from the middle of the section where the posted speed limit was corrected to 45 mph in 2008, from the former 40 mph.  Data is taken at Points 1 and A on the MDOT Traffic Control Order Map.

Survey Date Sep 2006 Aug 2008
Posted Speed Limit 40 MPH 45 MPH
% of Vehicles Obeying Speed Limit 33% 71%
50th Percentile Speed 43 MPH 43 MPH
85th Percentile Speed 47 MPH 47 MPH
90th Percentile Speed 49 MPH 49 MPH
% of Vehicles at 50 MPH or Higher 8.4% 8.6%
Fastest Speed Recorded 55 MPH 54 MPH
Total Range of Speeds 29 to 55 MPH 33 to 54 MPH
Maximum Difference in Speed 26 MPH 21 MPH

History Of Speeds on Washtenaw Avenue, Near the City Club
Data is from the middle of the section where the posted speed limit was corrected to 40 mph in 2008, from the former 30 mph.  Data is taken at Points 5 and P on the MDOT Traffic Control Order Map.

Survey Date Sep 2006 Aug 2008
Posted Speed Limit 30 MPH 40 MPH
% of Vehicles Obeying Speed Limit 8% 86%
50th Percentile Speed 35 MPH 36 MPH
85th Percentile Speed 40 MPH 40 MPH
90th Percentile Speed 41 MPH 42 MPH
% of Vehicles at 45 MPH or Higher 0.7% 1.7%
Fastest Speed Recorded 47 MPH 49 MPH
Total Range of Speeds 28 to 47 MPH 28 to 49 MPH
Maximum Difference in Speed 19 MPH 21 MPH

History Of Speeds on Washtenaw Avenue, Ann Arbor, Michigan
Data is from the middle of the section where the posted speed limit was corrected to 45 mph in 2008, from the former 35 mph.  Data is taken at Points R & S on the MDOT Traffic Control Order Map.

Survey Date Sep 2006 Aug 2008
Posted Speed Limit 35 MPH 45 MPH
% of Vehicles Obeying Speed Limit 4% 79%
50th Percentile Speed 42 MPH 43 MPH
85th Percentile Speed 47 MPH 46 MPH
90th Percentile Speed 48 MPH 47 MPH
% of Vehicles at 50 MPH or Higher 4.7% 2.6%
Fastest Speed Recorded 58 MPH 52 MPH
Total Range of Speeds 33 to 58 MPH 34 to 52 MPH
Maximum Difference in Speed 25 MPH 18 MPH

DEFINITIONS:

50th Percentile: Speed at which 50% of vehicles are above that speed and 50% are below.

85th Percentile: Speed at which 85% of the vehicles are below or right at that speed.

90th Percentile: Speed at which 90% of the vehicles are below or right at that speed.

Update (8/24/08) – The story was picked up by the media in Ann Arbor:

Speeding ticket challenge upheld in Washtenaw County Circuit Court
A better traffic flow? Ann Arbor man says so


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1,549 Responses to “The Effect Of Speed Limits On Actual Travel Speeds”

  1. [...] Understanding Highway Crash Data Montana: No Speed Limit Safety Paradox The Effect Of Speed Limits On Actual Travel Speeds __________________ A nation of sheep will beget a government of wolves. – Edward R. [...]

  2. JimSamsung says:

    3) Why after increasing the limit that the faster drivers either drive slower or at the same speed. Name the person that is in the 90% that does not increase speeds when they can? No one that I know. My guess is that there is a speed limit crack down that is holding the faster drivers down. I have done a lot of reading on the subject and after every speed limit increase there is a speed limit crackdown.
    4) Where did they come up with the numbers in the survey? how many people do you see that drive as little as 33 mph in a 45 mph zone? None that I ever see unless they are hitting their brakes or accelerating from a stop. How about that 29 mph car? I bet I could find a car driving 10 mph if they are accelerating from a stop. I guess that would look too obvious they are throwing the numbers in this report.
    5) Why is there such a huge difference between the 50% and the 90%. That does not look like any increase in safety to me.




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