Reader Response #4: The 85th Percentile Speed Rationale is Archaic

by | August 10, 2017 | Vision Zero | 7 comments

Bike in Philly

Img: Pew Charitable Trusts

Let’s say you’re a parent, and you have five kids. Then, let’s say four of those five kids tells you their favorite food is Oreo cookies. The fifth kid says they like grilled chicken, kale, carrots, and fruit.

Would you put all your kids on a strict diet of nothing but Oreos? Probably not.

And would you set all nutritional guidelines according to the kids who are super into Oreos?* Ha!

But that’s how speed limits were once determined, and how some people think they still should be—even in cities.

A search through the Bicycle Coalition of Greater Philadelphia’s blog comments finds more than 30 comments critical of the Bicycle Coalition’s activism for safer speeds throughout Philadelphia, citing something called the 85th Percentile. This may come as a shock (or not!), but the 85th Percentile approach is outdated and should be ignored.

The 85th Percentile idea, based on the 1964 “Solomon Curve” says speed limits should be set at what 85 percent of drivers think is healthy. It was created back when the highway system was still young, cars didn’t approach speeds as quickly as they do today, and we didn’t have the sort of statistics and research on traffic dangers we do today.

This percentile approach has helped create America’s culture of shrugging in the face of death. More than 40,000 people died in motor vehicle crashes in 2016 throughout the United States, costing the country and its people $432 billion. And 4.6 million people were injured on America’s streets. 2016 saw the most traffic deaths in the United States since 2007.

Speed is the leading cause of traffic deaths, and speeding makes it more likely the person involved in the crash will get injured once the crash occurs.

That’s why groups like the Bicycle Coalition – dedicated to making streets safe for all road users – have worked so hard for commonsense legislation like red light and speed cameras in Philadelphia.

I have long trashed the 85th Percentile speed approach as outdated and never meant for cities. That hasn’t stopped some — who feel motor vehicle users should be able to drive as fast as they want — from lashing out at the Bicycle Coalition’s rational attempts to curb speed and make streets safer for everyone.

And the Bicycle Coalition of Greater Philadelphia is not alone. In a huge step toward greater road safety for all, the National Transportation Safety Board released a new study identifying speed-related deaths as an “urgent and under-addressed national problem” in July, calling for an end to the 85th Percentile standards.

Among their specific recommendations: “Revise traditional speed-setting standards to balance 85 percentile approaches with safe systems approach that better incorporates crash history, safety of pedestrians, bicyclists.” Makes sense, considering speed-related deaths are three-times as likely on local roads.

The 85th Percentile rule aims to “accommodate speeding drivers, rather than reduce crashes,” as noted by Streetsblog.

“In general, there is not strong evidence that the 85th percentile speed within a given traffic flow equates to the speed with the lowest crash involvement rate,” the NTSB says. “Alternative approaches and expert systems for setting speed limits are available, which incorporate factors such as crash history and the presence of vulnerable road users such as pedestrians.”

*I cannot take credit for that analogy. I first read it on Rebecca Sanders’ Twitter feed. Thanks Rebecca!

UPDATE: Several organizations have put out press releases about the NTSB’s landmark study. I’ve added some links below. It’s important that cities and municipalities be able to reduce speed now.

NACTO: Federal Study Concludes U.S. Must Change Policies to Curb Epidemic of Deaths Caused by Excessive Motorist Speed

“U.S. streets have long been designed to promote speed at all costs, with deadly consequences,” said Linda Bailey, Executive Director of NACTO. “NTSB’s report is an urgent wake-up call for all walks of government to treat excessive speed as the deadly epidemic that it is. With safe street designs, automated enforcement practices, and policies that prioritize safety over speed, our streets can be modern, inviting places that make cities great places to be.”


“The National Complete Streets Coalition thanks NTSB for taking aim at the critical issue of speeding-related crashes that injure and kill far too many Americans each year — including many on foot or bike. We hope this report will continue to bring attention to the important yet often overlooked role of speeding in traffic injuries and fatalities for everyone who uses our streets…The Coalition is committed to continuing dialogue around speeding-related injuries and fatalities. Thank you to the NTSB team for spotlighting the issue of speeding and traffic safety.”

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  1. Daron Phillips

    Matthew Johnson

    President at Johnson Traffic Design

    A friend of mine commented about an article he read recently about how the 85th percentile speed is, well… the title of the article should clue you in:

    One for the Dustbin: The 85th Percentile Rule in Traffic Engineering

    The article was written by a young lady who,m according to her bio-blurb “is a newspaper reporter-turned planner/advocate who manages the [no free advertising here] from glamorous Cleveland, Ohio. She also writes about urban issues particular to the industrial Midwest at [no free advertising here].

    The author does seem to be quite prolific in her writing blogging. I saw that she has written posted several hundred articles over the past few years. Her article post centered around her discussion with the local traffic engineer who did not seem to really know what he was talking about: That’s when the local traffic engineer explained to me that the 35 mph speed limit – which, in effect, means drivers can go 45 with impunity – was appropriate for my street because 15 percent of drivers exceed 35 mph. Instead of doing more research, she found an article blogpost that tries to discredit the use of the 85th percentile because… well I am not really sure why. It apparently has something to do with drivers not being reasonable and prudent. He also wrote (this is an exact duplication of what he blogged) “AND mixing anthropological assumptions with pseudo-science. Not generally regarded as a good idea.” By the way the blog was written by an environmental engineer from Portugal working for a firm in Copenhagen that “specialise in bicycles as transport”. I guess we don’t need to voir dire their qualifications for speed analysis.

    I won’t keep going on about the obvious bias in both of those articles blogs. I will say that they both made me laugh when I read their assumptions and their speed zoning bona fides.

    Once my laughing subsided I realized that the articles did make me think and question our methods for setting speeds. I wondered if the curve in the manuals showing the cumulative percentages of speeds was still valid. Now I have my own counting equipment and I recently completed a study where I collected some speed data on a decently traveled road (ADT just under 10,000).

    I went to my trusty computer and pulled up the speed data and exported the raw speed data to Excel and began looking at the data. I reviewed the data in the exact same way I would with a real speed study (I have done quite a few of these in my career, in fact I was called about doing one today).

    I think I should probably explain what I am talking about with the 85th percentile. The 85th percentile speed is defined as the speed at which, or below, 85% of road users believe is safe. This number is found by measuring speeds on a roadway in several locations along the corridor. The data we use is always during an off peak period and we only measure vehicles that are not being affected by any other vehicles. Through many many many statistical studies, an incredibly steep transition has been found in speed data. The transition begins in the 10%-15% of drivers range and ends in the 85% to 90% range. This range of data covers only a change in speed of 15 mph but covers up to 85% of drivers.

    The chart above is the actual graph I made of the speed data I collected for my study. The green shading is simply the area of the curve between 45 mph and 60 mph. The roadway where this data was collected was posted 55 mph and dropped to 50 mph about 100′ to the west of this counting station.

    The data points show that those 15 mph capture 79% of the drivers on that roadway. If we increase the envelope by 5 mph in either direction, we do not gain many more drivers. In fact, to capture all of the drivers, we would have to increase the envelope more than 25 mph in either direction and we would only gain 21% of drivers. That means that 21% of drivers on this roadway cover 50 mph of speeds, where 79% of drivers are covered by just 15 mph.

    The real reason why the 85th percentile speed has had such longevity is because it makes sense. If you set the speed limit too low, you will have compliance issues. If you set it too high, you have safety issues.

    We all have experienced a street where the speed limit was set artificially low. How did it make you feel? You see almost no traffic, 12′ wide lanes, you can see for almost half a mile and you feel comfortable on this straight piece of well-designed roadway but the posted speed limit is only 30 mph. You feel like you could easily hit 60 mph on this highway-like arterial. I can tell you how you would feel, just like me, you would feel frustrated and you cannot wait to get off this street an on to a street with a proper speed limit.

    Yes, the 85th percentile speed is real. Yes, it is statistical. And yes, it is the safest way to set the speed limit.

    One of the biggest fallacies that both of the bloggers above are a part of is that speed limits are for cyclists or pedestrians. They are not. Speed limits are for motorized vehicles and a properly speed zoned roadway is safer than one which has an artificially low speed limit.

    To be fair, I am a cyclist, not a road warrior mind you but an average cyclist, and I ride my cycle on the roadway. However, I do understand that I am sharing the road with a friend who can move faster than I could ever hope too even though that friend has more than 2,000 pounds on me. I recognize that I must respect my friend and his limitations when it comes to being safe around me. I must be proactive with my safety. After all, I am the one riding the lightest vehicle I can find that offers me absolutely no protection and wearing very little protective clothing while I am trying to occupy some of the space that my 2,000 pound friend would prefer to occupy. I am responsible for my safety, no one else.

  2. Jym Dyer

    ☼ Civil engineering after WWII overbuilt the roads, with the best of intentions. The technique was to survey an area for things like density, use, number of children, etc. and decide on a safe speed limit, then widen the road to add a margin of error. This margin of error is one standard deviation above the mean.

    Unfortunately, this is not a static system. Widen the road a bit, and motorists feel comfortable going faster, thereby erasing the margin of error and killing tens of thousands of Americans every year. What was supposed to be the speed limit is now the mean, and a new statistical distribution around these mean gives you the 85th percentile.

    As a result, we get post hoc justifications of the 85th percentile as “sensible” and “statistical,” but really it is just a statistical consequence of making the roads too wide. The original justification for the speed limit (density, usage) hasn’t made it a higher speed “sensible” or “statistical,” though, except in cases where speeding cars have ruined communities that used to be walkable.

  3. John Baxter

    I agree with the writer directly above, and there are many things that are being ignored by those who claim the 85th percentile speed is archaic, not the least of which is that speed is by far not the cause of all collisions between autos and pedestrians and bicycles–or even the most common cause, as calculated by many detailed studies. What Vison Zero proponents fail to see is that, if you have a heavily-traveled road and 15% of the people are moving faster than the 85th percentile speed, that provides more than enough vehicles and drivers to more than account for ALL the car-pedestrian and car-bicycle collisions that can be traceable to speed (about 21% according to a major New York study). Engage in practices that will drop the speeds of those well above the 85th percentile speed to that speed, and you will get the results your want. And, I for one, won’t oppose your efforts! Lower postings are not the answer, but a procedure that will affect only the guilty. Interesting statistic: Even a detailed New York study, provably lacking in objectivity toward autos, shows that separating pedestrian and bicycle crossing times from the green light for cars will reduce serious and deadly such crashes MORE than speed control. Vision Zero is as much anti-car as pro-safety, in its most extreme form. At least, that is the message you are sending to those of us who agree in principle with many of your goals, and who see significant benefits in many Vision Zero features, but who merely wish the science and engineering to make the decisions rather than the extremist, political thinking that seems to motivate too much of Vision Zero rhetoric.

  4. John Baxter

    It’s interesting to me that the original thoughts above equate kids’ appetites with responsible adults driving. If there ever were an unfair and completely failed comparison, that is it! Kids don’t vote, but we trust adults to choose public officials because most learn how to behave and think clearly. Think about it! Further, to state that most deadly crashes result from speed is NOT true when it comes to car-pedestrian crashes, as determined by two extremely thorough studies. Still further, we could probably eliminate all serious crashes and deaths by simply driving 15 mph everywhere. Is that an appropriate solution? The NTSB has adopted a very extreme, ultra-conservative position in this present quest. A Volstead Act on wheels. What ever happened to the concept of solving the problem in the least intrusive and fairest way? I’m not against safety, but these decisions result from an over-simplification of all the problems, as well as a bureaucratic approach that seeks to solve problems in a heavy-handed manner that shows no consideration for what it is actually like to drive in the real world, and no attempt whatever to search for the most creative and least intrusive solutions.

  5. James C. Walker

    Please explain one thing, and this is a serious question. If the 85th percentile speed in an area with significant cyclist and pedestrian traffic is 36 mph – regardless of whether the posted limit is 25, 30, 35 (85th method limit) or 40 mph – WHY do some people think it is safer to have the limit posted at 25, with NO difference in the actual travel speeds?

    (This is an actual case in East Lansing, near the Michigan State University Campus.)

    James C. Walker, National Motorists Association

  6. Alan Fody

    James, in the example you cited, if a motorist makes a mistake and hits a pedestrian at 25mph the pedestrian will likely live. If the motorist hits the pedestrian at 36mph they will likely die. Clearly the authority that sets the speed limits in East Lansing cares about protecting human lives, maybe the motorist should consider slowing down.

  7. James C. Walker

    Hi Alan Fody,
    Perhaps you missed a critical part of the question. The ACTUAL 85th percentile speed is 36 mph on that portion of Grand River Avenue (M-43) when the posted limit is correctly set at 35 mph. The ACTUAL 85th percentile speed was 36 mph when posted at 25 mph and enforced almost daily by East Lansing police for revenue. WHY is it safer to post 25 when the change in the actual 85th percentile travel speeds is 0 mph?

    I understand the obvious physics argument, but posted limits have almost no effect on the actual 85th percentile speeds (+ or – 0 to 3 mph). That is the part that is so hard to teach. If the state government (they own the road) wants to reduce 85th percentile speeds to about 25 mph, they would have to degrade the roadway environment so that now 85% of the drivers feel safe and comfortable only at speeds up to 25 mph. Heavy enforcement for decades had NO effect. But traffic calming that disturbs the normal traffic flow is almost never appropriate on main collector and arterial streets that carry the bulk of commuting, shopping and commercial traffic. Note that the crash rates remained the same whether posted at 25 or 35, per state police data.

    James C. Walker, National Motorists Association


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