The 85th Percentile Rule Is Killing Us

Photo: Anthony Quintano/Flickr
Photo: Anthony Quintano/Flickr

Traffic deaths in the U.S. are mounting, reaching more than 40,000 last year, and, according to a recent draft report by the National Transportation Safety Board, speed is the overlooked factor.

The NTSB reported that speeding accounts for about 10,000 deaths a year — as many as drunk driving. One of the agency’s key recommendation was to change the way streets are designed by reforming the “85th percentile rule,” a laissez faire approach that seeks to accommodate motorist behavior instead of engineering streets for safety.

It’s an argument that Randy LoBasso at the Bicycle Coalition of Greater Philadelphia has been making for a long time. Now the NTSB report is vindicating advocates’ critique of the 85th percentile rule, he writes:

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.

I have long trashed the 85th Percentile speed approach as outdated and never meant for cities.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.

Among [the NTSB’s] 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.”

“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.”

More recommended reading today: Pricetags shares some insight from transportation economist Todd Litman about the under-appreciated transportation costs of buying a house in the suburbs. Greater Greater Washington considers the potential drawbacks of diverting cut-through car traffic away from residents streets. And the Raleigh Connoisseur reports that a major increase in bus service goes into effect this week in Wake County, following a November vote to increase the local sales tax half a cent.

  • Rational Person

    People on this blog have good intentions but are very badly misinformed! I want to rename the title to this section as thus: “Newton’s laws are killing us. Sir Isac Newton proposed a theory on gravitation in the 17th Century when an apple allegedly struck him on the head…This theory was shown to be invalid by Albert Einstein when he showed that Newton’s laws do not apply near the speed of light…if we allow the 85th percentile rule to set speed limits, the posted speed will creep up to the speed of light and then irresponsible drivers will attempt to drive 5 MPH faster than the speed of light!”

    I will cut to the chase: you will not achieve your goal of creating a safer environment for people on bicycles or pedestrians by lowering posted speed limits from the 85 percentile of free flowing traffic under good conditions. What you will do is provide a false sense of security to people on bicycles and pedestrians while at the same time providing political cover to insurance companies and local courts to continue ripping off the public at best and/or deliberately subjecting the public to policies that harm/obstruct reaching better or more robust safety goals at worst.

    There are lots of bad laws on the books that should be eliminated and the manner we do speed enforcement in this country is one of the exhibit A’s. Why? Well, here are some facts that people need to get their heads around 1) Just because you set a speed limit on a sign, it does not necessarily follow that most drivers will use that number to set their operating speed! The fact is, it is mainly the environment that drivers are operating in that has a bigger influence on driving speed than speed limit signs. In fact, speed limit signs on local roads are a really weak way of setting speeds or warning of hazards along the road. The factors that determine driver operating speeds are: a) number of access points, b) width of roadway, c) visibility, d) presence of pedestrians and bicycles, e) smoothness of the road. Each roadway has a natural speed that most drivers will go at that has more to do with the way the road is constructed than with an arbitrarily low number posted on a speed limit sign. 2) There is an assumption that if you get officer Friendly to pass out speeding tickets when the speed limits are posted significantly below the 85th percentile like he is passing out candy to babies, that will deter people from speeding. This is a really false assumption that is not supported by real world experience. There are two things that happen when you attempt to enforce under posted speed limits: 1) make insurance companies and local courts richer from the revenue, 2) reduce the smooth flow of traffic which actually results in more accidents not fewer (there is a bifurcation in the speeds drivers drive at: some drivers follow the posted limit while most drivers ignore an under-posted limit resulting in a wide variance in speeds which correlates to a higher accident risk.)

    If you truly want to create a friendly and safer environment for bicyclists and pedestrians by lowering speeds, you need to focus on the construction of the roadways. You need to focus on more robust use of various traffic calming techniques to encourage drivers to go slower (reducing posted speed limits is a purely snake oil solution to a very real problem.)

  • Dissenting Voice

    I want to rename the title to this section as thus: “Newton’s laws are killing us. Sir Isac Newton proposed a theory on gravitation in the 17th Century when an apple allegedly struck him on the head…This theory was shown to be invalid by Albert Einstein when he showed that Newton’s laws do not apply near the speed of light…if we allow the 85th percentile rule to set speed limits, the posted speed will creep up to the speed of light and then irresponsible drivers will attempt to drive 5 MPH faster than the speed of light!”

    I will cut to the chase: you will not achieve your goal of creating a safer environment for people on bicycles or pedestrians by lowering posted speed limits from the 85 percentile of free flowing traffic under good conditions. What you will do is provide a false sense of security to people on bicycles and pedestrians while at the same time providing political cover to insurance companies and local courts to continue ripping off the public at best and/or deliberately subjecting the public to policies that harm/obstruct reaching better or more robust safety goals at worst.

    There are lots of bad laws on the books that should be eliminated and the manner we do speed enforcement in this country is one of the exhibit A’s. Why? Well, here are some facts that people need to get their heads around 1) Just because you set a speed limit on a sign, it does not necessarily follow that most drivers will use that number to set their operating speed! The fact is, it is mainly the environment that drivers are operating in that has a bigger influence on driving speed than speed limit signs. In fact, speed limit signs on local roads are a really weak way of setting speeds or warning of hazards along the road. The factors that determine driver operating speeds are: a) number of access points, b) width of roadway, c) visibility, d) presence of pedestrians and bicycles, e) smoothness of the road. Each roadway has a natural speed that most drivers will go at that has more to do with the way the road is constructed than with an arbitrarily low number posted on a speed limit sign. 2) There is an assumption that if you get officer Friendly to pass out speeding tickets when the speed limits are posted significantly below the 85th percentile like he is passing out candy to babies, that will deter people from speeding. This is a really false assumption that is not supported by real world experience. There are two things that happen when you attempt to enforce under posted speed limits: 1) make insurance companies and local courts richer from the revenue, 2) reduce the smooth flow of traffic which actually results in more accidents not fewer (there is a bifurcation in the speeds drivers drive at: some drivers follow the posted limit while most drivers ignore an under-posted limit resulting in a wide variance in speeds which correlates to a higher accident risk.)

    If you truly want to create a friendly and safer environment for bicyclists and pedestrians by lowering speeds, you need to focus on the construction of the roadways. You need to focus on more robust use of various traffic calming techniques to encourage drivers to go slower (reducing posted speed limits is a purely snake oil solution to a very real problem.)

  • Ann Arbor Resident

    The title to this section should read: “Newton’s laws are killing us. Sir Isac Newton proposed a theory on gravitation in the 17th Century when an apple allegedly struck him on the head…This theory was shown to be invalid by Albert Einstein when he showed that Newton’s laws do not apply near the speed of light…if we allow the 85th percentile rule to set speed limits, the posted speed will creep up to the speed of light and then irresponsible drivers will attempt to drive 5 MPH faster than the speed of light!”

    I will cut to the chase: you will not achieve your goal of creating a safer environment for people on bicycles or pedestrians by lowering posted speed limits from the 85 percentile of free flowing traffic under good conditions. What you will do is provide a false sense of security to people on bicycles and pedestrians while at the same time providing political cover to insurance companies and local courts to continue ripping off the public at best and/or deliberately subjecting the public to policies that harm/obstruct reaching better or more robust safety goals at worst.

    There are lots of bad laws on the books that should be eliminated and the manner we do speed enforcement in this country is one of the exhibit A’s. Why? Well, here are some facts that people need to get their heads around 1) Just because you set a speed limit on a sign, it does not necessarily follow that most drivers will use that number to set their operating speed! The fact is, it is mainly the environment that drivers are operating in that has a bigger influence on driving speed than speed limit signs. In fact, speed limit signs on local roads are a really weak way of setting speeds or warning of hazards along the road. The factors that determine driver operating speeds are: a) number of access points, b) width of roadway, c) visibility, d) presence of pedestrians and bicycles, e) smoothness of the road. Each roadway has a natural speed that most drivers will go at that has more to do with the way the road is constructed than with an arbitrarily low number posted on a speed limit sign. 2) There is an assumption that if you get officer Friendly to pass out speeding tickets when the speed limits are posted significantly below the 85th percentile like he is passing out candy to babies, that will deter people from speeding. This is a really false assumption that is not supported by real world experience. There are two things that happen when you attempt to enforce under posted speed limits: 1) make insurance companies and local courts richer from the revenue, 2) reduce the smooth flow of traffic which actually results in more accidents not fewer (there is a bifurcation in the speeds drivers drive at: some drivers follow the posted limit while most drivers ignore an under-posted limit resulting in a wide variance in speeds which correlates to a higher accident risk.)

    If you truly want to create a friendly and safer environment for bicyclists and pedestrians by lowering speeds, you need to focus on the construction of the roadways. You need to focus on more robust use of various traffic calming techniques to encourage drivers to go slower (reducing posted speed limits is a purely snake oil solution to a very real problem.)

  • Hugh Janus

    Wrong John! Raise the posted speed limit to the 85th percentile and then measure the 85th percentile speed and they are about the same. Under-post the limit significantly below the 85th percentile and have officer Friendly issue tickets until he is blue in the face and guess what? The 85th percentile speed will be about the same. The construction of the road and surrounding environment determine the speeds people drive, not speed limit signs for most drivers. If you under-post speed limits, you end up with a wider variance in speeds since some drivers are afraid of getting a ticket and follow the posted limit; this results in less uniform traffic flow which correlates to higher risk and more accidents.

  • Hugh Janus

    John, the hazardous conditions mitigation you mention above should not be encoded into the speed limit; you should post an advisory sign to warn drivers of the hazard. This is because speed limit signs are not effective at controlling speeds!

  • John Eldon

    Then let’s narrow the travel lanes, paint prominent crosswalks, install some mild speed humps in residential zones, etc. Let’s really try to calm the traffic.

  • John Eldon

    Neither are advisory warning signs.

  • John Eldon

    Undivided rural 2-lane highways are the more dangerous roads in the country. I don’t mind 75 on a properly divided limited-access freeway, but high speeds are incompatible with roads with pedestrian, bicyclist, and/or driveway access.

  • John Eldon

    The facts, such as the relationship between collision survivability and vehicular speed, say otehrwise.

  • Hugh Janus

    Yes, you need to focus on traffic calming. Insurance companies and your local police are selling snake oil. Cities don’t want to do this because speed enforcement goes from a profit center to a cost center when you actually design roads to get the speed down.

  • John Eldon

    My own city (Encinitas) is very actively pushing traffic calming, because we fortunately have a council that value public safety over citation profit.

  • Hugh Janus

    My city, Ann Arbor, MI does not value human life and likes money from ticket revenue. We have under-posted speed limits galore. It is not uncommon for posted limits to be in the 20-30th percentiles of free flowing traffic or less. The city is expanding their revenue generating racket into poorly engineered pedestrian cross walks that are sure to cause conflict between drivers and pedestrians.

  • Hugh Janus

    Encinitas, CA? CA has very progressive speed laws on the books. They refer to the 85th percentile speed as the critical speed and require it to be measured every 5 years or the officer issuing a ticket is an incompetent witness. Giving people the tools to fight the racket is one way to force the focus on real engineering solutions that actually work.

  • How do you see bus riders only paying 25% of the cost of operating the bus as paying one cent for roads? Your 25% doesn’t even cover operating costs exclusive of driver wages.

    Car drivers obviously live somewhere and they pay local taxes as well as sales taxes too. If you ride your bike outside your taxing jurisdiction you pay nothing unless you buy something.

    I will agree that neither fuel taxes nor bottom 98% wages have kept pace with actual inflation, not the heavily-doctored and omissive version our government reports.

    According to my own research into retail price inflation, on an average national basis, using 50 common readily-comparable consumer items, retail price inflation since 1968 was up by 1403.5% in November, 2016.

    At 1403.5% our non-tipped minimum wage should be $22.65 per-hour and the tipped minimum $16.14 per-hour just to have kept parity on retail purchasing power since 1968, which would include no fair compensation for an average 40% increase in worker productivity since 1968 either.

    According to a recent report from the Federal Reserve, US median home price is up by 335% since January, 1986, while in actual numbers, median income is only up by 90%. The same report found that median home price here in Metro-Denver was up by 404.5% since January, 1990.

    I would venture to guess that neither local tax, State tax, fuel tax, sales tax, liquor tax, nor income tax have kept pace with actual retail price inflation over any time frame of more than a couple of years

    So before taxes can be raised, wages have to be fairly raised first, or raising taxes will subtract from other spending. My income in 1982 was $30K and to have parity on just average retail purchasing power today would have to be $135K, which would include nothing for productivity gain nor an additional 35 years of experience either.

    As the most I made since 1982 was $63K, regardless of how much road maintenance or other infrastructure costs have risen, in my case here in Denver, my income is only up by 110% in 35 years in actual numbers doing the same job, while housing and medical care is up by more than 500%.

    By how much more than 110% can taxes be raised over the last 35 years without cutting into other spending, with the cost of housing and medical care up by 500% or better since 1982?

    Almost everyone including highway and mass transit funding is seeing a slow bleed-away of purchasing power thanks to our Republicans, who steadfastly fail to fairly raise wages or taxes, while also supporting offshoring as many middle-class jobs as possible too, cutting into our national standard of living.

  • Even the average urban grocery store requires 6-8 truckloads per-day. Trailer length of 48 feet and overall length of 65 feet became legal in 1982 and 53-foot trailer length became legal in 1986, with overall length rising to as much as 72 feet.

    In my opinion the people you need to blame are Republicans and Clinton establishment Democrats for offshoring or otherwise destroying as many middle-class jobs as possible, replacing them with low-paying service industry jobs, and for failing to fairly raise wages and taxes too.

  • The biggest problem I see in that photo is that the telephone pole is just a foot from the road. Poles should never be that close to roads, it is too-dangerous. How about putting a guardrail up to deflect errant vehicles back to the road?

    On any truck or bus route minimum lane width needs to be 11 feet just to allow two buses to pass without hitting their mirrors or having to stray across the center-line. Furthermore, State and Federal oversize vehicle laws may have to be changed in-order to narrow travel lanes on truck routes too.

  • If 80% of people drive cars, 9% ride public transit, 3% ride bicycles, and the rest either walk, work from home, or are home-bound or institutionalized, what percentage of roadway funding should go to making bicycling safer, considering what bicyclists pay to ride on roads?

  • Japan lost an entire bullet train 100% fatal in their tsunami.

  • Joe R.

    Didn’t happen: http://wonderfulrife.blogspot.com/2012/12/the-mystery-of-lost-shinkansen.html

    It took me all of one minute to find that.

  • And that is what is wrong this site. as it tries to pass itself off as an urban transportation planning site then readers like you are angry at the 80% of Americans who drive, plus everything you buy comes by truck too. I am angry myself as I pay for our roads and for the time savings involved too.

  • Jeffrey Baker

    That must have been a hell of a bender to keep you off the air for three months. Get help.

  • LinuxGuy

    Being that bicyclists pay nothing to ride on the roads, zero seems about right. Want bike services, start paying for them.

  • LinuxGuy

    Proper speed limits prevent crashes, so if they do not occur that is best.

  • Hugh Janus

    “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.” Except the policies you propose actually make the streets less safe for everyone. I have a question for you, do you intend to make insurance companies and local courts rich? Because when you start moving the posted limit below the 85th percentile, whether it is a rural or urban area, the only thing that improves is the revenue to the aforementioned entities. Under-posting limits marginally increases the accident risk to motorists with no improvement in safety to pedestrians or bicyclists. Thanks for helping to sell snake oil instead of real, practical solutions.

  • Hugh Janus

    “Road geometry, sight distance, the presence or absence of medians and median cuts, driveway accesses, protected turn lanes, etc.” Robert, these are the same things that correlate to the 85th percentile speeds. My hometown of Ann Arbor, MI improved the road near my home by making it wider and smoother so as to better accommodate bicycle lanes. The result was that the 85th percentile speed went from around 41 to 48 MPH on the 1/2 mile of improved road.

  • Michael

    My thought on the matter is that both sides of the aisle have bought into the mythology that suburban bargain shopping will generate cultural & financial wealth. As opposed to, say, hard work or making meaningful investments into the future. The fact that sprawl has produced so little of civic & cultural value while housing the vast majority of US wealth at the peak of our financial strength, so just how bad this idea has been.

    On the logistics of the grocery store, the issue is the excessively wide turn radius and the fact that the damage the trucks do increases at an exponential function of weight (so a 20 ton truck does more than 2x the damage of the 10 ton truck). Those cost are carried by the municipality, but really only provide value to a handful of businesses in town and none of the citizens. It’s a give away to entice a certain type of business (and one that historically is relatively exploiting to its workers and supply chain) at the competitive cost to all other business that don’t operate at that (ridiculously large) scale. It’s also a big reason why it’s hard to start a competitive business when then entry point is seemingly 25k square feet and 2 loading docks.

    On the exporting of jobs, I think these local decisions grease the wheels on the whole thing. Industry follows supply chains. When we radically redesigned the way we build communities with gusto from about 1980 to 2010 toward sprawl (of course we were dabbling for years before but this was the height) we created a regulatory & financial system where the best place to do business was by a highway off ramp far away from water, especially freshwater. The result is that highway rich, water poor places grew remarkably during the period. And industry moved overseas as its much harder to moved a truck from Port of LA to
    Kansas City as from Pennsylvania to KC. When goods couldn’t flow freely on freeways to any corner of the US, connections to rail matter at the manufacturer (something no Chinese company has.. )

    But I believe these conditions will ultimately be pretty temporary as our financial ability/willingness to maintain roads at a level necessary to keep this model going fails. These places are not lovable; i just don’t see there being the cultural willingness to keep maintaining each of those (350k) intersections in front of each store. I.e. We’ll probably say, we’ll keep the road, but that light and those turn lanes are your problem – which will be a dagger to an already fragile model.

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