How Should Speed Limits Be Set?

February 29th, 2008 Posted in Speed Limits

By Bonnie Sesolak, NMA Development Director

speedlimitsign “Realistic” speed limits should invite public compliance by conforming to the behavior of normal responsible drivers. This results in the safest and most efficient use of streets, roads, and highways.

Traffic engineering research has determined that speed limits should be established according to the 85th percentile of free flowing traffic. This means the limit should be set at a speed that 85 percent of people are driving at or below.

Numerous studies have shown that the 85th percentile is the safest possible speed limit. It recognizes that most drivers voluntarily adjust their speed to the total roadway/roadside environment (width, alignment, surface condition, roadside development, pedestrian activity, weather, light conditions).

It is not appropriate to set a limit below the 85th percentile to accommodate factors that are obvious to road users. Drivers are already taking these factors into account with their existing speed decisions.

Main Factors Used to Set Speed Limits:

Engineering Study
Speed zones are established on the basis of an engineering study. The engineering study includes an analysis of the current speed distribution of free- flowing vehicles.

85th percentile speed
The speed at or below which 85 percent of the sample of free flowing vehicles are traveling. This speed should be determined by conducting a speed survey following the procedure contained in the Manual of Transportation Engineering Studies.

Advisory speed limits
These should be implemented when certain features, such as sharp corners, require a momentary reduction of speed.

Additional Notes:

Each speed zone should be periodically restudied to determine that the established speed limit is appropriate. The suggested maximum interval is five years.

In addition, an engineering study should be conducted whenever there is a change in the roadway that would affect the prevailing speed. Such changes would include elimination of parking, added lanes, signal coordination and changes in roadside development.

According to an Institute of Transportation Engineers Study, those driving 10 mph slower than the prevailing speed are six times as likely to be involved in an accident.

For more information, studies, and articles on speed limits, visit the speed limit section of the NMA’s website.

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  1. 13 Responses to “How Should Speed Limits Be Set?”

  2. By HOWIE on Mar 1, 2008

    EVERY TIME I SEE CONSTRUCTION OF A NEW TRAFFIC LIGHT SYSTEM AT SOME GOD-FORSAKEN INTERSECTION,, I CAN’T HELP BUT WONDER WHICH POLITICIANS HAVE THEIR DIRTY MITTS IN ON THE COMPANY(S) WHO SUPPLY THOSE DAMN THINGS.
    SEEMS TO BE A VERY LUCRATIVE BUSINESS TO BE IN ON. AND WHO BETTER THAN TO STEER OUR MONEY IN THAT DIRECTION. WONDER IF ANYONE HAS EVER DELVED INTO THAT SITUATION???

  3. By Carol on Mar 2, 2008

    How do you go about getting limits increased? On both the main roads I travel to work daily, most cars are going 10mph above the posted limit. Can a local agency or gov’t office be contacted? If so, which office? Do they have to honor the request to re-evaluate the posted limits?

  4. By V LOUIS on Mar 2, 2008

    TRAFFIC SAFETY BULL
    SAVING LIVES BULL
    PROTECTING THE PUBLIC BULL
    WHAT ELSE IT IS ALL BULLSHIT

    SIMPLY THESE AND ALL OTHER ‘TICKETS’ ARE PROFIT CENTERS FOR THE CITIES/TOWNS/STATE, THE FEDS HAVE YET TO GET IN ON THIS WINDFALL.
    YOU FIGURE IT OUT AT ABOUT $50.00 + A TICKET AN OFFICER DOES ANYWHERE FROM 10 TO 50 A DAY THAT IS $500.00 TO $5000.00 A DAY PER OFFICER(WHEN THEY COULD BE SOLVING REAL CRIMES) MINUS THEIR SALARY YOU ARE LOOKING AT A LOT OF PROFIT FOR TOWNS ETC. WAIT TILL THEY GET LIKE GERMANY (AT LEAST IN THE TICKET DEPARTMENT ONLY) THEY CHARGE SLIDING FINES ACCORDING TO THE WEALTH OF THE OFFENDER HA HA HA HA. GERMANY HIGHEST SPEED LIMITS IN THE WORLD LOWEST ACCIDENT/DEATH ON THE ROADS. GO FIGURE

    BUT DONT FORGET ‘PROFIT CENTER’

  5. By V LOUIS on Mar 2, 2008

    SORRY FOR THE DOUBLE POST BUT I FORGOT.

    WHE I DRIVE 50 - 55 MPH ON HIGHWAYS OR PARKWAYS I TEND TO NOD OFF YOU KNOW FALL ASLEEP, IT IS A REAL SRUGGLE TO STAY AWAKE. ITS HARD TO DRIVE WHEN IT IS COLD WITH ALL WINDOWS OPEN TO TRY AND STAY AWAKE.

    BY THE WAY THERE WAS A STUDY DONE YEARS AGO THE RESULT WAS THIS ‘WHEN TRUCKERS ARE DRIVING THE OPEN ROAD AT 55 MPH THEIR BRAIN IS IN THE CLOSEST STATE TO SLEEP AS IT CAN BE WITHOUT ACTUALLY SLEEPING’

  6. By P Candida on Mar 2, 2008

    On average 45000 die in traffic accidents a year in the US. What I would like to see is a device for cars so that it regulates the speed in residential, business and retail districts to 25 mph.

  7. By Schwinn on Mar 3, 2008

    @P Candida: Besides the fact that your post is totally irrelevant to an article about HIGHWAY speed limits: Slower does not always mean safer. Maybe if people stop jaywalking everywhere, and maybe if people actually LOOK BOTH WAYS before crossing, we’d have fewer accidents, too? But, there is no money in enforcing those laws.

    Anyway, regarding the article - I’d love to see more references to this “six times” value that is mentioned in the article. In fact, I advise that you post more references to the actual articles in the future. Now, I realize that this particular one has not been published, but I can’t find any reference at the ITE site, nor at the linked article with NMA headers, to the “6-times” value that is provided in the article…

  8. By darrel on Mar 21, 2008

    I am a LEO, and I can tell you that speed has a lot to do with traffic accidents.

    I am not just talking about people going over the speed limit, but those going too fast for conditions. Higher speeds mean less reaction time, less control, and more distance covered in the event a hazard presents itself. I agree, there are areas where the speed limit is incredibly slow. Generally though, officers are aware of the situation and don’t punish the public for poorly marked speed zones. In fact, Myself and other officers that I currently work with, have complained about poorly marked speed zones or enforcement practices. There has to be a standard, or some will take advantage and cause more dangerous driving environments.

    You must also remember not everyone has the same driving skills or sense. Some drivers are better or worse than others. And in regards to the 85th percentile formula that the article speaks about, try this formula in an area that doesn’t get worked by law enforcement for an extended period of time and see what “safe” speeds the public drives. Its been my experience that most drivers on highways drive at or close to the speed limit. These areas are worked routinely by law enforcement though. If you take law enforcement out of the picture and do your 85th percentile formula to leave it up to the drivers to determine the appropriate speed, you will find that many will drive faster than is safe for conditions, and cause more dangerous driving environments.

    Not everyone anticipates an animal running out in front of them, how many intersections there are, or what’s over the next hill. Serious injury accidents will also increase. I dont need a statistic to tell me that, I have seen it with my own eyes. The only exception to this is obvious conditions, such as weather.Though many speed related accidents occur during inclement weather, when you compare that number to how many drivers are on the road, generally the majority of the public does a good job of policing themselves in driving an appropriate speed.

    You will never have a speed limit system that is perfect for everyone.

  9. By Schwinn on Mar 21, 2008

    LEO or not, you are grossly misinformed. Traffic studies have already been done to show that 85th percentile speeds are SAFER than lower speed limits. Did you even bother to read the article?

    Keep in mind, the 85th percentile number is based on FEDERAL studies on HIGHWAYS. This can extend a little bit into major state routes, but no one ever says that this is totally applicable to city streets. So, your argument over intersections and such is moot. Please stay on topic.

    As for “anticipating an animal running out in front of them”… that’s silliness. Accidents WILL happen - you can’t eliminate ALL of them. Such is the nature of life. So, whether an animal pops out while you are doing 55 or 75 doesn’t really make much of a difference, since you simply will not be able to predict their actions, nor the ability of the driver to see the action any better at either speed.

    The bottom line is, take a look at the highway fatality rates before and after the 55mph NMSL was repealed… people who believed 55 was safer said we would have “thousands more deaths” per year and that the roadways would become extremely dangerous. Guess what - it didn’t happen. Why? Because it’s nonsense.

    Sure, you, being an LEO, see only the bad side of accidents, but do you get to see the millions of cars that go by without incident? You need to put these things in perspective, and look at them in the larger picture. Your one or even 10 datapoints are irrelevant in the grand scheme of things, and making a judgement on the entire population based on such a tiny slice of evidence (that is biased because it’s your job to look at accidents) is hardly a scientific study.

  10. By Joe on Mar 21, 2008

    Schwinn you drive a good point. I’ve wondered about this for a long time. I work for a major air carrier and we had a bad crash many moons ago. As part of the investigative team the CO. picked several from our maintenance facility. One was a inspector mechanic. After he finished his assignment and came back on normal duty he was so traumatized it took him several months before he could start doing his job again. He was so afraid to sign anything off. I believe this “human factor” might be a problem as it pertains to traffic enforcement. It may cause an officer to over enforce as a result of some trauma he/she received as a result of working a bad accident. Therefore I’ve always felt that if possible the enforcers and the accident team should be two different parties.

    And your right about the zillion miles driven annually across this country under all kinds of conditions with all kinds of equipment and frankly I’m surprised we don’t have more accidents. If all you see are accidents all day long your thinking is going to trend toward this as the norm instead of the exception.

    As you so eloquently stated, we must look at the facts and quit making rules and enforcement based on emotions. Lets start with a first class drivers education course for our youth. Something more along the lines of what the Germans do.

    And for darrel’s sake in my driving envornment left lane hogs form a whole lot worse safety scenario then speeders but that’s not what gets enforced. It’s the easy pickings, the low hanging fruit …..speeders. On one of these blogs, when confronted, one LEO explained that he believed in his patrol area that rear-enders in rush hour was probably the most prevalent accident he observes. It’s unlikely someone is going to be speeding when creeping along in rush hour traffic. Lets face it, the ratio of accident causation to speeding citations is way out of proportion. Fact is radar just don’t find bad drivers.

  11. By James Young on Mar 21, 2008

    darrel writes:
    { Generally though, officers are aware of the situation and don’t punish the public for poorly marked speed zones. In fact, Myself and other officers that I currently work with, have complained about poorly marked speed zones or enforcement practices.}

    You have no idea how rare that is. The overwhelming majority of LEOs working traffic just trigger the radar, flash the lights and write the citation.

    { And in regards to the 85th percentile formula that the article speaks about, try this formula in an area that doesn’t get worked by law enforcement for an extended period of time and see what “safe” speeds the public drives. Its been my experience that most drivers on highways drive at or close to the speed limit. These areas are worked routinely by law enforcement though. If you take law enforcement out of the picture and do your 85th percentile formula to leave it up to the drivers to determine the appropriate speed, you will find that many will drive faster than is safe for conditions, and cause more dangerous driving environments.}

    Commonly, approximately half the drivers on the highway drive over the posted limit because the 50th percentile (median) is usually very near the posted limit. A critical characteristic of the 85th percentile is that it is calculated in the absence of enforcement, that is all those drivers that you accuse of near suicidal behavior DON’T do those things. Further, there is emerging engineering data that lead us to believe that on Interstate-grade and rural highways, the 95th percentile is the optimal and safest speed.

    {Not everyone anticipates an animal running out in front of them, how many intersections there are, or what’s over the next hill.}

    Interstate-grade roadways don’t have intersections and rarely do animals intrude. The risk of a meteor hitting the roadway is very real but we should not build public policy around it.

    {Serious injury accidents will also increase. I dont need a statistic to tell me that, I have seen it with my own eyes.}

    Your own statistics do not support that assertion, which has been made by the anti-destination league for about 40 years.

    Speeds, drivers, vehicles and roads are all increasing; injury crashes are decreasing.

  12. By Schwinn on Mar 22, 2008

    James Young writes:
    {darrel writes:
    { Generally though, officers are aware of the situation and don’t punish the public for poorly marked speed zones. In fact, Myself and other officers that I currently work with, have complained about poorly marked speed zones or enforcement practices.}

    You have no idea how rare that is. The overwhelming majority of LEOs working traffic just trigger the radar, flash the lights and write the citation.}

    Well said. What’s more, under such a system, leaving the decision on whether to give or not give a ticket up to the officer leads to an unfair situation and to discrimination and emotion. If it’s REALLY unsafe, then just give the ticket - period. The fact is, that it’s NOT unsafe. Yet, you pulled them over… why? Just to harass the driver into “being good”? Do you honestly think that this works, when it’s proven that even the ticket itself isn’t a deterrent to speeding?

    The fact is, it’s not about safety - it’s about making money. I don’t disagree that LEOs are underpaid, overall, for a dangerous line of work. However, fleecing the public while sitting on the side of the road, and trying to make quotas on speeding tickets has NOTHING to do with safety… it’s all about the money.

    Anyway, back to the original point, let’s look at the original argument in a different way: On the highway, what causes accidents? Well, by definition, it’s when one car hits another. Aside from vehicle failure-related accidents, and inattentive drivers sideswiping each other, or skidding out of control. In the latter case, speed has little to do with the problem, unless the car was hydroplaning, in which case EVERYONE wouldn’t be doing it. In the case of hitting an ice patch, no amount of speed reduction can replace attentive-ness to the road conditions, etc. Yet, how often do you see speeding tickets in adverse conditions? No, it’s usually on the sunny, clear, warm days that police hand out tickets. Again, it’s not about the safety, otherwise you’d see more tickets in the winter here in the north… but you don’t.

    So, ignoring those cases, accidents happen when two (or more) cars were going at different speeds in order to hit each other (front to rear). These are the most common accidents on the highways. Put another way, if everyone drove the same exact speed, we wouldn’t have these types of accidents, because it’s simply impossible. Again, how often do you see people getting tickets for driving too slow? If one person is driving slow on the highway, I guarantee that one of the faster cars will get the ticket instead. Heck, the radar gun doesn’t even show you the speed of the slowest vehicle… yet that person is likely the MOST unsafe! (Samuel C. Tignor and Davey Warren. “Driver Speed Behavior on U.S. Streets and Highways.” Institute of Transportation Engineers: 1990 Compendium of Technical Papers, 1990 August, p. 85.
    “The accident involvement rates on streets and highways in urban areas was highest for the slowest 5 percent of traffic, lowest for traffic in the 30 to 95 percentile range and increased for the fastest 5 percent of traffic.”)

    Ok, so what about the onramps and offramps? You have to slow down for those, right? Very true. This is one of the reasons that the left lane is for passing, and the right is slower for exiting/entering. Again, not so many tickets for lane-laws, are there?

    Additionally, on/off ramps have speedup/slowdown sections to allow the cars to speed up and slow down as necessary OFF the highway. Yet, I know in Massachusetts, many such lanes are horribly maintained, horribly short, or simply missing. What’s worse, there are STOP signs on them, totally defeating the purpose of the lane in the first place! Even when there isn’t a stop sign, many dangerous drivers stop AT THE END of these lanes, or simply refuse to speed up to traffic speeds before merging. Again, not many tickets out there for these people, are there?

    So, as a win-win, I propose to the LEOs that they go after the unsafe acts that we see every day, and ticket those people more often. That way, you get to continue making your money, AND you make the roads safer. Heck, if you want to be totally lazy, pick an onramp, and just watch the dumb people stopping at the end of it… you don’t even have to move to give them a ticket, and don’t need to point a laser or radar gun at all. I guarantee you can make TONS of money this way, but more importantly, you’ll make it safer for the people on the highway. What a concept.

    In other words, there are plenty of ways to improve safety that are going COMPLETELY overlooked.

    Lastly, just to reinforce James’ statement above, as well as mine: Keep the emotion out of it, and look at the real data. Speeding does NOT cause more accidents. 85% speed limits are totally safe in most highway cases, because, as you’ll note, most of the people are driving at that speed… and as I mentioned above, if everyone drove at the same speed…

  13. By Pirate News TV on Mar 27, 2008

    Translation: The faster you drive, and the faster everyone drives, then the higher the speed limit. Traffic engineering surveys don’t count when under the influence of a speed trap. So when you see a cop, speed up! “Necessity” for safety is always a valid legal defense.

  14. By Schwinn on Mar 28, 2008

    By definition, a traffic study states that it cannot be done during any adverse situation, such as a police presence or construction.

    I am sure that most traffic studies are performed properly, but it’s the politics and brainwashing with “speed kills” that sets speed limits artificially low, which is more of a safety hazard than proper speed limits (as proven EVERY DAY since the repeal of the national speed limit.)

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