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Old Posted Aug 10, 2020, 6:17 PM
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M II A II R II K M II A II R II K is offline
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It's Time To Reconsider How Speed Limits Are Set

Why Slower Commutes Can Be A Good Thing

Aug 5, 2020

By Tiffany Chu

Read More: https://www.forbes.com/sites/tiffany.../#263ae42f1ce5


The reality is that speed limits on most U.S. streets are not chosen based on what speed is safest for everyone using the roadway they are set by the speed at or below which 85 percent of the motorists drive on a given road, an outdated guideline promoted by the Federal Highway Administration (FHWA). Today’s speed limits are not set for our safety. They are, in fact, making us less safe.

- When following the 85th percentile guideline, engineers recommend a speed limit that falls near the 85th percentile of the average speed naturally observed on a roadway. The method assumes two things: 1) that drivers pick a speed that is safe for the road conditions and 2) that it is safest to drive the speed desired by fellow drivers. Say you live along a state route in a rural area. If 85% of drivers drive at or below 55 mph, 55 mph would become the starting speed limit for said road. Engineers can then make adjustments to the starting speed limits based on neighborhood factors such as physical design of the street or volume of pedestrian traffic. — Researchers from University of California Institute of Transportation Studies point out that there is no empirical study demonstrating that the 85th percentile speed optimizes safety, but the guideline is difficult to get around because of its presence in the FHWA’s Manual on Uniform Traffic Control Devices (MUTCD) and promotion by the Institute of Transportation Engineers (ITE). For decades, transportation engineers routinely follow guidelines outlined by the MUTCD and ITE as standard practice.

- Simply lowering speed limits may not be enough, though. Drivers don’t sufficiently reduce their speed unless the roadway is designed and engineered to make them slow down. The 85th percentile guideline was actually born from this knowledge, if speed limits are reduced too far below the average speed, crashes can increase because drivers stop paying attention. Policy makers and advocacy groups are proposing new paths forward. The National Association of City Transportation Officials (NACTO) just released a recent report, City Limits, which calls for designing slow zones, along with setting default speed limits on many streets at once, and using conflict density and activity level to set corridor speed. — The critique of the 85th percentile is not new, advocates have long called for its removal. What is new is broad-based support and awareness as evidenced by NACTO’s City Limits report, and government initiatives such as CalSTA’s Zero Fatalities Task Force. These efforts are major steps forward to unwind this harmful practice of speed-setting, and reorient around neighborhood and design considerations.


Examples of engineering measures to slow vehicular speeds for safer streets. Peter Biczók & Kelsey Jones, Remix

Additional examples of engineering measures to design more inclusive streets - for people of all ages and abilities, using a variety of modes. Peter Biczók & Kelsey Jones, Remix

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Old Posted Aug 10, 2020, 7:46 PM
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Busy Bee Busy Bee is offline
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Never been a fan of speed humps or tables. All they seem to result in is drivers doing there normal over amount, then slamming on the brakes to go over the hump and then gunning it right back, and repeat. Narrowing lanes and tactile pavements would go a long way.
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Old Posted Aug 11, 2020, 12:04 AM
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ardecila ardecila is offline
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This is the tail wagging the dog. The 85th percentile rule isn't really the problem, street design is. If you provide a street where drivers automatically slow down due to design factors - narrow width, lots of obstacles, etc - then you don't really need the speed limit. You still have to set one, but it just serves as a justification for punishing the most reckless drivers who refuse to pay attention to the design cues.

America is full of terribly designed "stroads" that encourage very fast speeds by virtue of their drag-strip style design. The 85th-percentile rule recognizes the reality of these roads and attempts to legalize the behavior of MOST drivers who are only reacting rationally to the infrastructure that is provided to them. But getting rid of the 85th-percentile rule and setting the speed limits lower won't reduce the average speed, it'll just let cops write more tickets.
la forme d'une ville change plus vite, hélas! que le coeur d'un mortel...
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Old Posted Aug 11, 2020, 3:08 PM
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Steely Dan Steely Dan is online now
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Originally Posted by Busy Bee View Post
Never been a fan of speed humps or tables.
I agree that speed bumps aren't perfect, but I sure was happy to see the city install two massive axle-busting speed bumps on our 700' long one-way side street block last summer

Now idiots can't haul-ass down it at 40mph.
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Old Posted Aug 11, 2020, 6:11 PM
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People often make the argument about emergency vehicles being hindered by these traffic-calming measures.

I think more STOP or YIELD signs can be just as effective.
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Old Posted Aug 11, 2020, 10:06 PM
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I think that traffic calming can be done to an extent (we have lots of roundabouts in residential streets here in West Seattle, and I think that they are effective), but I think that better traffic enforcement would have a greater effect.

Last edited by SFBruin; Aug 12, 2020 at 4:48 PM. Reason: Grammar.
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Old Posted Aug 12, 2020, 8:00 PM
mrnyc mrnyc is offline
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its even more overdue to take out those epic azz wide streets in many long post peak downtowns, like detroit and cleveland for example. replace with wider sidewalks, bike lanes, transit. that will slow the already long reduced traffic in and of itself.
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