James will have a field day with this one...

[GoCougs]

Wait, so you're against gov't intervention to prevent stupid people from doing stupid things, yet you want the gov't to limit how fast we drive because people are stupid?

You're contradicting yourself here. 

Not at all - people can drive as fast or as stupid as they want on their own private property; when people are on the public roads however with me and mine, they will (and should be) subject to the rule of law as it pertains to driving.

[NomisR]

Not at all - people can drive as fast or as stupid as they want on their own private property; when people are on the public roads however with me and mine, they will (and should be) subject to the rule of law as it pertains to driving.

Yeah, and we're saying the rule doesn't make the road safer but more dangerous.  Having artificially low speed limit means it reduces max flow of traffic allowed on a road, increases traffic density, increases travel time, and increases chance of accident.  And I'm stuck with those assholes who can't drive a lot longer.  I don't see any benefits for me.

[GoCougs]

Yeah, and we're saying the rule doesn't make the road safer but more dangerous.  Having artificially low speed limit means it reduces max flow of traffic allowed on a road, increases traffic density, increases travel time, and increases chance of accident.  And I'm stuck with those assholes who can't drive a lot longer.  I don't see any benefits for me.

Yes, I understand but I see an ulterior motive to want legal sanction to drive faster, not make the roads safer.

[Soup DeVille]

Yes, I understand but I see an ulterior motive to want legal sanction to drive faster, not make the roads safer.

Motives are irrelevant.

[GoCougs]

Motives are irrelevant.

It's human nature that when motives are shown to be false that which they underpin is rejected.

[Soup DeVille]

It's human nature that when motives are shown to be false that which they underpin is rejected.

What if the underpinning is not motivation, but actual, you know: data.

[GoCougs]

Data is not engineering though.

[Soup DeVille]

Data is not engineering though.

But law making is?

[GoCougs]

But law making is?

Laws based on engineering, yes.

[Soup DeVille]

Laws based on engineering, yes.

Which you think our current speed laws are?

[hotrodalex]

Current speed laws are based on politics. Like all laws.

[bing_oh]

The 85th percentile rule is proven wrong between November and March each year in Ohio. Poor road conditions prove consistantly that the average driver has difficulty accurately judging conditions and adjusting their speed accordingly.

[TurboDan]

The 85th percentile rule is proven wrong between November and March each year in Ohio. Poor road conditions prove consistantly that the average driver has difficulty accurately judging conditions and adjusting their speed accordingly.

Also, if the 85th percentile rule were enforced around here, all our speed limits would be about 20 miles lower. We have tons of senior citizen developments springing up all over the place. 

[James Young]

The 85th percentile rule is proven wrong between November and March each year in Ohio. Poor road conditions prove consistantly that the average driver has difficulty accurately judging conditions and adjusting their speed accordingly.

That has nothing to do with the 85th percentile concept.  If limits were set at the 85th percentile (or the newly emerging 95th percentile concept for interstate-grade roads), actual travel speeds would not change significantly but most drivers would then be compliant.  And LE could then concentrate on the dangerous exceptions instead of the low-hanging fruit of the 65% of drivers exceeding a 35th percentile limit, all on a highway with a design speed of 100+ mph. 

[GoCougs]

Which you think our current speed laws are?

Do you mean physics rather than engineering?

[GoCougs]

That has nothing to do with the 85th percentile concept.  If limits were set at the 85th percentile (or the newly emerging 95th percentile concept for interstate-grade roads), actual travel speeds would not change significantly but most drivers would then be compliant.  And LE could then concentrate on the dangerous exceptions instead of the low-hanging fruit of the 65% of drivers exceeding a 35th percentile limit, all on a highway with a design speed of 100+ mph. 

It illustrates that many people don't understand the physics involved; i.e., open-loop behavior of the whole is not a proper yardstick.

100 mph freeways? Not a chance.

[dazzleman]

Also, if the 85th percentile rule were enforced around here, all our speed limits would be about 20 miles lower. We have tons of senior citizen developments springing up all over the place. 

Good thing you're there to bring up the average speed, Dan....

[Byteme]

That has nothing to do with the 85th percentile concept.  If limits were set at the 85th percentile (or the newly emerging 95th percentile concept for interstate-grade roads), actual travel speeds would not change significantly but most drivers would then be compliant.  And LE could then concentrate on the dangerous exceptions instead of the low-hanging fruit of the 65% of drivers exceeding a 35th percentile limit, all on a highway with a design speed of 100+ mph. 

Actually it has everything to do with it.  The 85th percentile rule simply says set the speed limit at the speed that seperates 85% of the drivers from the fastest 15%.  Now if the majority of drivers happen to driving too fast for conditions (weather, traffic density, whatever), then that speed limit, set using the 85th percentile is too high.  Using the 85th percentile rule assumes drivers drive the maximum speed, and don't exceed the speed, at which they are competent to drive.

[Tave]

The 85th percentile rule is proven wrong between November and March each year in Ohio. Poor road conditions prove consistantly that the average driver has difficulty accurately judging conditions and adjusting their speed accordingly.

Current speed limits are often too fast for inclement weather, and motorists who drive at that speed can be ticketed accordingly.

[NomisR]

Actually it has everything to do with it.  The 85th percentile rule simply says set the speed limit at the speed that seperates 85% of the drivers from the fastest 15%.  Now if the majority of drivers happen to driving too fast for conditions (weather, traffic density, whatever), then that speed limit, set using the 85th percentile is too high.  Using the 85th percentile rule assumes drivers drive the maximum speed, and don't exceed the speed, at which they are competent to drive.

Wouldn't the speed limit change accordingly with the conditions of the road rather than just a set limit? 

[Byteme]

Wouldn't the speed limit change accordingly with the conditions of the road rather than just a set limit? 

In an ideal world yes, but how would one design a system like that?  Sensors that feed electronic signs that lower the speed limits when it rains of snows?  Some states already have lower limits at night. 

In theory the posted speed limit is suppose to represent the safe speed under ideal road and environmental conditions.  Clear day, good visibility, dry pavement, moderate traffic, etc. 

No one apparently has come up with a system that make everyone happy, nor is anyone likely to.    

[NomisR]

In an ideal world yes, but how would one design a system like that?  Sensors that feed electronic signs that lower the speed limits when it rains of snows?  Some states already have lower limits at night. 

In theory the posted speed limit is suppose to represent the safe speed under ideal road and environmental conditions.  Clear day, good visibility, dry pavement, moderate traffic, etc. 

No one apparently has come up with a system that make everyone happy, nor is anyone likely to.    

Yeah, so people getting in accidents in bad weather really has nothing to do with speed limit since there are people that still drive the speed limit when you have visibility of 100 ft with no head lights on.  There's always exceptions to the rule. 

[Tave]

In an ideal world yes, but how would one design a system like that? 

How would it be materially different from our current system?


Road has 75 mph limit

It snows

Cop tickets motorist for driving 75 mph on the slick road

[Eye of the Tiger]

Around here, they have 45 MPH signs that start flashing when road conditions are bad. About half of the driver's heed them, so it results in a large differential of traffic speeds - some drivers going 35-45, others going 60-70.

[James Young]

Gentlemen:

The 85th percentile is a concept used by traffic engineers; it is also a rule outlined  in MUTCD, the Manual of Uniform Traffic Control Devices, a federal law which all 50 states are obligated to follow even if they were not signatories (all are in some stage of signing off on the newest version of MUTCD, which is updated regularly). 

The 85th percentile speed is determined when certain conditions are met.  Those conditions are:
?   Performed on average week day at off-peak hours
?   Under favorable weather conditions
?   Include only ?free floating? vehicles, i.e., with a minimum amount of headway or distance between them; in a pack, only the lead car (the slowest) is counted.
?   Include a minimum of 125 valid readings
?   Discontinued after 2 hours if radar is used, 4 hours if a traffic counter/classifier is used
?   Trucks and busses are recorded separately
?   Must be conducted in the absence of enforcement
?   Must be conducted at specified minimum distances from control devices such as stop and yield signs and stop lights.

If the conditions do not obtain, then the concept has no meaning and driver judgment takes over.  Poor judgment under adverse conditions has nothing to do with the determination of the 85th and its subsequent recommendation by engineers to politicians, who then usually ignore it. 

Finally, speed limits as purveyors of useful information is often touted by the safety cabal ? which believes drivers are too stupid to make decisions for themselves ? but have proved over the years that they are infamously poor sources of information since so many of them are wrong and drivers know this.  A highway marked at 70 mph with an actual 85th percentile speed of 86 mph is a lie, i.e., it provides false information.  Drivers know this and behave accordingly.