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Old 03-08-2013, 08:16 PM
 
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The problem is, engineers design roads to be as straight and wide as possible, regardless of context. Then drivers naturally respond by driving faster, because straight and wide is the context which is most relevant to them. But there are other things that matter when determining speed limits; things which the engineers ignored when designing the road. Thus the 85th percentile idea is BS, and it's certainly not legally binding, as Ram2 suggests when he says:

Quote:
The 55 limit is illegally underposted for revenue generation purposes. Speed limits are supposed to be set based on 85th percentile measured vehicle speeds, as measured by traffic engineers.
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Old 03-08-2013, 09:40 PM
 
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While there are different ways speed limits can be set, the 85% is part of the engineering approach to setting them. The following is a snippet from the Federal Highway Association on setting speed limits.


Operating Speed Method

Most engineering approaches to speed limit setting are based on the 85th percentile speed—the speed at which 85 percent of free-flowing traffic is traveling at or below. The typical procedure is to set the speed limit at or near the 85th percentile speed of free-flow traffic. Adjustments to either increase or decrease the speed limits may be made depending on infrastructure and traffic conditions.

Setting a speed limit based on the 85th percentile speed was originally based on safety. Specifically, research at the time had shown that traveling at or around one standard deviation above the mean operating speed (which is approximately the 85th percentile speed) yields the lowest crash risk for drivers. Furthermore, crash risk increases rapidly for drivers traveling two standard deviations or more above or below the mean operating speed. Therefore, the 85th percentile speed separates acceptable speed behavior from unsafe speed behavior that disproportionately contributes to crash risk.*

The 85th percentile speed method is also attractive because it reflects the collective judgment of the vast majority of drivers as to a reasonable speed for given traffic and roadway conditions. This is aligned with the general policy sentiment that laws (i.e., speed limits) should not make people acting reasonably into law-breakers. Setting a speed limit even 5 mph (8 km/h) below the 85th percentile speed can make almost half the drivers illegal; setting a speed limit 5 mph (8 km/h) above the 85th percentile speed will likely make few additional drivers legal.

Under the operating speed method of setting speed limits, the first approximation of the speed limit is to set the speed limit at the 85th percentile speed. The MUTCD recommends that the speed limit be within 5 mph (8 km/h) of the 85th percentile speed of free-flowing traffic. The posted speed limit shall be in multiples of 5 mph15, or 10 km/h for jurisdictions that employ metric.22

While the MUTCD recommends setting the posted speed limits near the 85th percentile speed, and traffic engineers say that agencies are using the 85th percentile speed to set speed limits, in reality the speed limit is often set much lower. At these locations, the 85th percentile operating speeds exceed the posted speed limits; and, in many cases, the 50th percentile operating speed is either near or exceeds that posted speed limit as well.16 Many agencies deviate from their agency's written guidelines and instead post lower speed limits. According to an ITE Engineering Council Technical Committee survey, these reduced speed limits are often the result of political pressures.17 However, it is important to note that setting speed limits lower than 85th percentile speed does not encourage compliance with the posted speed limit.

So,basically the 85% is based on data, although some argue the data are old. Speeds can be adjusted from that starting point. As pointed out, governments can adjust speed limits lower based upon any criteria they want including speed traps andother political reasons I'm not saying they always do this, but I'm sure in some cases they do.
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Old 03-08-2013, 10:06 PM
 
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I wasn't arguing the 85th percentile method isn't used. Just that it's not mandatory.

I also alluded to the idea that it's not always rational. You could design a road through a pedestrian area that feels perfectly fine to drive 45+ on, but where a 45 limit creates many more fatalities than a 25 limit. Or where people are discouraged to walk or bike in a neighborhood because the road feels okay to drive fast on.

Part of this can be cured, and actually I would say it's a better solution in many if not most cases than changing the speed limit, by engineering the road to be less "efficient" (i.e. less wide and straight). But that's anathema to American civic engineering culture. This needs a paradigm shift, desperately.
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Old 03-09-2013, 03:45 PM
 
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Quote:
Originally Posted by natininja View Post
I wasn't arguing the 85th percentile method isn't used. Just that it's not mandatory.

I also alluded to the idea that it's not always rational. You could design a road through a pedestrian area that feels perfectly fine to drive 45+ on, but where a 45 limit creates many more fatalities than a 25 limit.

You do not design roads for a given speed limit. You post speed limits based on the maximum allowable safe speed for a given road.
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Old 03-09-2013, 03:50 PM
 
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Quote:
Originally Posted by bertwrench View Post
While the MUTCD recommends setting the posted speed limits near the 85th percentile speed, and traffic engineers say that agencies are using the 85th percentile speed to set speed limits, in reality the speed limit is often set much lower. At these locations, the 85th percentile operating speeds exceed the posted speed limits; and, in many cases, the 50th percentile operating speed is either near or exceeds that posted speed limit as well.16 Many agencies deviate from their agency's written guidelines and instead post lower speed limits. According to an ITE Engineering Council Technical Committee survey, these reduced speed limits are often the result of political pressures.17 However, it is important to note that setting speed limits lower than 85th percentile speed does not encourage compliance with the posted speed limit.

Ohio is the biggest offender of using underposted speed limits for revenue generation purposes. Prior to 1974, you could legally and safely drive 75mph on the Ohio Turnpike. Notice how the current speed limit is underposted?

The 55mph speed limit had zero compliance.
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Old 03-09-2013, 04:03 PM
 
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Originally Posted by ram2 View Post
You do not design roads for a given speed limit. You post speed limits based on the maximum allowable safe speed for a given road.
Safe for whom? Drivers? Children walking to school? Motorcyclists? How many fatalities are acceptable on a stretch of road before a speed limit is decreased? How many injuries? How much intimidation or perception of danger is acceptable for non-motorists, on neighborhood streets? Near bus stops serving the working poor and disabled? Near schools?
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Old 03-09-2013, 04:16 PM
 
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Quote:
Originally Posted by natininja View Post
Safe for whom? Drivers? Children walking to school? Motorcyclists? How many fatalities are acceptable on a stretch of road before a speed limit is decreased? How many injuries? How much intimidation or perception of danger is acceptable for non-motorists, on neighborhood streets? Near bus stops serving the working poor and disabled? Near schools?

Fatalities on American highways decreased when speed limits were increased after elimination of the 55 limit. That's right - speed limits up/accidents down.
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Old 03-09-2013, 09:02 PM
 
Location: Chicago(Northside)
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Quote:
Originally Posted by ram2 View Post
Fatalities on American highways decreased when speed limits were increased after elimination of the 55 limit. That's right - speed limits up/accidents down.
I recently did a major research paper about this and you are correct.
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Old 03-09-2013, 10:09 PM
 
1,295 posts, read 1,526,872 times
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Quote:
Originally Posted by ram2 View Post
Fatalities on American highways decreased when speed limits were increased after elimination of the 55 limit. That's right - speed limits up/accidents down.
You say this as though it contradicts anything I said. It doesn't; you're fighting a strawman. Of course the speed limit and the design speed of the road should be in alignment, but it doesn't mean designing a road for a higher speed is safer. You say roads should be designed for safety, not for a specific speed, but you misunderstand that these two things are inextricably linked. When highways are designed, they must have a generally consistent design speed, or conflict points are created all over. In order to achieve this, which is done for safety reasons, a specific design speed must be decided upon. Picking a lower design speed and setting the speed limit accordingly would make a safer environment.

Changing the speed limit on a road is a crude and inefficient way to increase safety (be that an interstate highway or residential street). Altering design elements to lower the speed drivers naturally drive on a stretch of road is very effective and preferred, but American engineers are not really there yet.

Last edited by natininja; 03-09-2013 at 10:27 PM..
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Old 03-10-2013, 04:35 PM
 
Location: Mason, OH
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People forget why the 55 MPH National Speed Limit was set in 1974. The purpose was to save gas and our dependence on foreign oil. Of course nobody liked it and enforcement was terrible. It was eventually done away with.

When the Interstates were brand new and nice and smooth a 70 MPH limit was very reasonable in normal traffic volumes such as in rural Ohio, and even higher in the western states with much lower traffic densities. But even then when in a conjested urban area the limit was reduced to 55, which I believe is very rational. Urban environments have shorter exit and entrance ramps due to space constrictions, shorter line of sight distances to evaluate what is going on, more lane changes and jockeying. Here is where I feel the 55 MPH limit still makes sense from a safety standpoint to permit longer reaction times.

There are many Interstates which I feel are unsafe at 70 MPH due to condition. The amount of wash-board roadbeds, cracked and uneven pavement, potholes, is everywhere. Our roads are simply not being maintained as a high speed roadway should be. If you constantly have to correct the suspension of the vehicle the speed is too high.
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