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RandallT

If a Road is Unsafe, Raise the Speed Limit

12 posts in this topic

No, seriously. This is counterintuitive, but it makes sense when you think about what we REALLY do when we drive (90% of us actually ignore the actual limit). In Michigan, they found that most roads had limits that were too low:

http://priceonomics.com/is-every-speed-limit-too-low/

Quote:
The answer lies in realizing that the speed limit really is just a number on a sign, and it has very little influence on how fast people drive. “Over the years, I’ve done many follow up studies after we raise or lower a speed limit,” Megge tells us. “Almost every time, the 85th percentile speed doesn’t change, or if it does, it’s by about 2 or 3 mph.”
...
Luckily, there is some logic to the speed people choose other than the need for speed. The speed drivers choose is not based on laws or street signs, but the weather, number of intersections, presence of pedestrians and curves, and all the other information that factors into the principle, as Lt. Megge puts it, that “no one I know who gets into their car wants to crash.”
...
Traffic engineers believe that the 85th percentile speed is the ideal speed limit because it leads to the least variability between driving speeds and therefore safer roads. When the speed limit is correctly set at the 85th percentile speed, the minority of drivers that do conscientiously follow speed limits are no longer driving much slower than the speed of traffic. The choice of the 85th percentile speed is a data-driven conclusion -- as noted Lt. Megge and speed limit resources like the Michigan State Police’s guide -- that has been established by the consistent findings of years of traffic studies.

Anyway, thought this was interesting.

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No, seriously. This is counterintuitive, but it makes sense when you think about what we REALLY do when we drive (90% of us actually ignore the actual limit). In Michigan, they found that most roads had limits that were too low:

http://priceonomics.com/is-every-speed-limit-too-low/

Anyway, thought this was interesting.


The other side of looking at this is to make sure people obey the speed limit. Least variability can also be achieved by following the traffic rules.

One issue with making an 85th percentile limit is that a "safe speed" is very subjective. Muscle and Sports car owners get used to moving pretty fast, and consider going 40mph a safe speed to travel through a neighborhood street. More people would opt to have faster cars. Following this logic, car manufacturers would not be limited in their design of the vehicles. More people would just get used to traveling faster speeds. Even though everyone is now traveling at the same speed things around them aren't. Pedestrians are still going 2-4mph, bicycles are going the same speed. Only the cars are moving faster.

Not sure who is behind the study, but they are clearly not considering all factors.

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The other side of looking at this is to make sure people obey the speed limit. Least variability can also be achieved by following the traffic rules.

One issue with making an 85th percentile limit is that a "safe speed" is very subjective. Muscle and Sports car owners get used to moving pretty fast, and consider going 40mph a safe speed to travel through a neighborhood street. More people would opt to have faster cars. Following this logic, car manufacturers would not be limited in their design of the vehicles. More people would just get used to traveling faster speeds. Even though everyone is now traveling at the same speed things around them aren't. Pedestrians are still going 2-4mph, bicycles are going the same speed. Only the cars are moving faster.

Not sure who is behind the study, but they are clearly not considering all factors.

Good points, but my take is that drivers already factor in the obstacles around them in determining their safe speed. It's a purely individual things and we mostly ignore signs.

Here's Michigan's detailed booklet on the topic. Some graphs and data to further justify their stance.

http://www.michigan.gov/documents/Establishing_Realistic_Speedlimits_85625_7.pdf

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I agree that many of us don't obey the speed limit, but here's an excerpt that stuck out to me:

"Consistently, the 85th percentile loses out to the perception that faster roads are less safe, so speed limits should be low. It’s a misconception, Lt. Megge says, that he faces often in his work. When he proposes raising a speed limit, the initial reaction is always “Oh my god! You can’t do that. People are already going too fast.” People think raising the limit 10 mph will lead people to drive 10 mph faster , when really changing the limit has almost no impact on the speed of traffic. "

Well, we raised the speed limits in CA about a decade ago from 55 everywhere to: 75 outside heavily populated areas; 70 for inter city traffic and 65 for local freeways. The average speed of traffic has gone up 10mph above the posted limits.

Local roads still have speed limits of 25,30,35,40,45,50 and 55, but the freeways are really fast when there are less cars on the road (i.e., not during "rush" hour).

I'm not going to admit to any of my driving habits, but I do drive safely and in line with everyone else on the road.

One day, when I had a lot of time to kill, I decided to go exactly 60mph (the posted limit). People were passing me pretty fast. I was on the right most lane too. In any case, my mpg rating went from my normal 27mpg up to to 37.5 mpg. I was pretty amazed that the same car going 60mph versus, ahem, my normal driving speed could attain that much more efficiency. Not sure what it would do to my 245hp engine (Just a normal car around here.), if I sustained this behavior. :whistle:

There was a definitive improvement in the efficiency, albeit a limited test. I don't even dare try 55mph, because I almost with certainty would be rear ended by a U-haul truck with the trailer attachment going significantly faster than the trailer speed limit of 55mph.

So, anyone thinking that people will not drive faster with faster speed limits does not understand human behavior too well.

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So, anyone thinking that people will not drive faster with faster speed limits does not understand human behavior too well.

This is all new to me, but I'm finding it fascinating. Here's a study:

http://www.motorists.org/speed-limits/effects-raising-lowering

Summary of Findings

The pertinent findings of this study, conducted to examine the effects of lowing and raising posted speed limits on nonlimited access rural and urban highways, are listed below. NMA's comments and clarifications are in [italics]

A review of the before and after speed data at each site revealed that differences in mean speeds, standard deviations of speeds, 85th percentile speeds, and other percentile speeds were generally less than 2 mi/h (3.2 km/h) and were not related to the amount the posted speed limit was changed.

When sites were grouped by the amount of speed limit change, the differences in percentile speeds for each group were less than 1.5 mi/h (2.4 km/h), irrespective of whether the speed limit was lowered or raised or the amount that the limit was changed. The average change in percentile speeds at sites where limits were lowered and at sites where speed limits were raised was less than 1 mi/h (1.6 km/h).

The small differences in before and after speeds were statistically significant due primarily to the large sample size collected.

At 34 locations, existing speed limits were posted within 5 mi/h (8 km/h) of the 85th percentile speeds. When speed limits at these sites were lowered more than 5 mi/h (8 km/h) below the 85th percentile speed, the mean difference in percentile speeds was less than 1 mi/h (1.6 km/h). [In other words, lowering the speed limit does not mean traffic will slow down.]

At 21 other locations, existing speed limits were posted more than 5 mi/h (8 km/h) below the 85th percentile speeds. When the agencies raised the limits to within 5 mi/h (8 km/h) of the 85th percentile speeds at these sites, the mean difference in percentile speeds was less than 1 mi/h (1.6 km/h) [In other words, raising the speed limit does not mean traffic will speed up.]

By defining driver compliance as the number or percentage of drivers that travel at or below the posted speed limit, major changes in compliance occurred when speed limits were raised or lowered. However, as reflected in small changes in vehicle speeds, driver behavior did not change, but the standard for measuring compliance, i.e., posted speed limit, changed. [Higher limits mean a greater number of drivers traveling at the limit instead of over it.]

Based on the free-flow speed date collected for a 24-h period at the experimental and comparison sites in 22 states, posted speed limits were set, on average, at the 45th percentile speed or below the average speed of traffic. [This means that, on average, only 45 percent of us abide by present speed limits.]

Only minor changes in vehicles following at headways of less than 2 s were found at the experimental sites with similar before and after traffic volumes. [People don't follow any closer when the limits are raised.]

The indirect effects of speed limit changes on a sample of five contiguous and adjacent roadways were found to be small and insignificant. [Changing a speed limit on one road does not mean that speeds on all roads nearby will change as well.]

There is not sufficient evidence, in this dataset, to reject the hypothesis that total crashes changed when posted speed limits were lowered more than 5 mi/h (8 km/h) below the 85th percentile speeds. [Lowering the speed limit does not reduce the number of crashes.]

There is not sufficient evidence, in this dataset, to reject the hypothesis that total crashes changed when posted speed limits were raised more than 5 mi/h (8 km/h) below the 85th percentile speeds. [Raising the speed limit does not increase the number of crashes.]

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This is all new to me, but I'm finding it fascinating. Here's a study:

http://www.motorists.org/speed-limits/effects-raising-lowering


It is interesting to hear all these case studies. One thing I am wondering is if the drivers keep in their mind that the limit is, say, 55mph, and they drive 65 thinking that cops should not stop them because they are going only 10mph or a hair less than the speed limit?

Perhaps having the limits limits the drivers consciously or otherwise to some number above the speed limit and that is why the number is 85% and not some other arbitrary number?

Interesting discussion.

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Yes I agree with the regard that changing the speed doesn't necessarily change what the drivers will do. Most people get comfortable with their roadways. Unless you travel out of town, you probably know the speed limits where you live at pretty well. I doubt many will even notice if a sign has changed because it's location probably wouldn't change, and it's still black and white coloring. As for accidents, I would agree that speed differential matters. The further away the speeds are the less time people have to react. Cars going nearly the same speed look like they move slower because of perspective. Yet increase in speed does decrease reaction time, that is pure physics. I will also say, changing the freeway speed limit from 65 to 75 probably wouldn't have an effect like changing a speed limit like 35 to 45. There is a natural comfort level people would drive at. I would presume that most people don't like to travel above 80 mph, especially after years of driving 10 mph lower. I would say it would be more comfortable to increase the speed at lower speeds because it still wouldn't feel too fast. I get their point that if the speed limit is bellow 85th then raise it because people were not paying attention to it anyways. I would say that is ok as long as the issue is causing accidents due to those actually going the speed limit. Meaning is the sign actually causing people to go bellow the 85th percentile speed when they wouldn't otherwise, and is this causing accidents. If so, then it becomes a safety issue. If not, then I say don't waste money to change it.

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Yes I agree with the regard that changing the speed doesn't necessarily change what the drivers will do. Most people get comfortable with their roadways. Unless you travel out of town, you probably know the speed limits where you live at pretty well. I doubt many will even notice if a sign has changed because it's location probably wouldn't change, and it's still black and white coloring.

As for accidents, I would agree that speed differential matters. The further away the speeds are the less time people have to react. Cars going nearly the same speed look like they move slower because of perspective.

Yet increase in speed does decrease reaction time, that is pure physics.

I will also say, changing the freeway speed limit from 65 to 75 probably wouldn't have an effect like changing a speed limit like 35 to 45. There is a natural comfort level people would drive at. I would presume that most people don't like to travel above 80 mph, especially after years of driving 10 mph lower. I would say it would be more comfortable to increase the speed at lower speeds because it still wouldn't feel too fast.

I get their point that if the speed limit is bellow 85th then raise it because people were not paying attention to it anyways. I would say that is ok as long as the issue is causing accidents due to those actually going the speed limit. Meaning is the sign actually causing people to go bellow the 85th percentile speed when they wouldn't otherwise, and is this causing accidents. If so, then it becomes a safety issue. If not, then I say don't waste money to change it.

This seems reasonable, and maybe even a little high, given the small roads in the mid-west.

Even out here in CA with larger freeways, the top median speeds are in the 85 range.

90+ gets a little iffy with the typical tire. You only see cars capable of high speeds on the 5 freeway going to the Bay Area.

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This seems reasonable, and maybe even a little high, given the small roads in the mid-west. Even out here in CA with larger freeways, the top median speeds are in the 85 range. 90+ gets a little iffy with the typical tire. You only see cars capable of high speeds on the 5 freeway going to the Bay Area.

Yeah, I think got my car up to 95 comfortably. I like my car, handles high speed well. My old 96 sable, it's steering wheel would shake if it got near 80. In ohio most speeds are sub 80 on the freeways. I tend to go 75-77 mph, and I pass about 95% of all drivers.

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I live in Michigan and it's not uncommon for I-96 to travel around 85 mph most of the time.   If everyone is traveling at that speed, it is safe.   What really causes problems is you'll get a person that has decided to save gas and drive 60 mph.   It doesn't take long for somebody to come upon a vehicle traveling 25 mph less than another.   You see severe braking, swerving in and out of traffic and a lot of middle fingers.     For some reason, the slow drivers (speed limit), feel empowered to drive in the fast lane and stay there.

It wouldn't matter the speed limit, the situation is the same.

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I talked to some older generation folks pre-55 limits, and their take was that 85 seemed like the normal freeway/highway speeds. Seems like 85mph is the magic number and not 85th percentile.

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