4 ms·

I found myself disagreeing with the premise that highway speed limits don't influence people's driving speed. I may simply be part of the 15%, but consider the

I found myself disagreeing with the premise that highway speed limits don't influence people's driving speed. I may simply be part of the 15%, but consider the following anecdata: On I-90 in upstate NY, the "speed limit" is 65. I've gone by several state troopers at 70-75 without issue or confrontation. I don't go faster, not because traffic dictates, but I'm concerned about litigious policemen. To put it in Freakonomic terms, I'm going just slow enough that I won't get caught. If the limit was 10 mph higher, I'd drive 10 faster, as would everyone who was on the road for more than one exit (or governed semi tractors). There is very little crowding on the stretches of highway I'm most familiar with. If the speed limit was 120, I'd probably cruise around 90 or so, assuming it wasn't rush hour. I propose that there is a diminishing impulse for travel rates as they increase, but also that on well-cleared, divided, limited-access highways, we have diminishing utility for speed limits as a regulating concept. Highway driving is the easiest driving to do. There are more variables in control than anywhere else. Why not re-evaluate the roads, not based on the 85th percentile of drivers living under the threat of litigation based on arbitrary bureaucrats, but on simple road capacity and volume?

mcdougle 12y ago
You do raise a good point. I know that, when I started driving, my dad taught me a neat trick: drive 8.5 mph over the speed limit, and you'll never get pulled over. That's (supposedly) because a ticket for anything less than 10mph is simply not worth it for the cops. If you go any more than 8.5mph over, though, you might accidentally cross that 10mph mark, and if a cop's around he'll get you. And, sure enough, any ticket I've ever gotten is because I was going over 10mph faster than the speed limit.
- incision 12y ago
  >'That's (supposedly) because a ticket for anything less than 10mph is simply not worth it for the cops.' Depends on your cops. I've witnessed tickets for 5 over (40 in a 45), 4 over (49 in a 45) and a stop, thankfully without a ticket for 1 over (31 in 30). I'm given to thinking of it as a lottery. Someone is getting pulled over, today me, tomorrow someone else. I'm just glad I haven't had a case of 'marijuana smell' for 10 years or so.
  - mnarayan01 12y ago
    I think it more depends on your jurisdiction. When I'm driving somewhere new, I'll make sure I'm not going faster than average (even if that's below the speed limit). In places that I'm more familiar with, I'll do 9 over.
  - mwfunk 12y ago
    I'm also in the "drive a little under 10 over" camp, and I totally accept that once every 10 years (or whenever), I'm going to be in the wrong place at the wrong time and get a ticket for it. I'm OK with that. It's not fair, but it's well below the bar of meaningful injustice for me to really care about it all that much.
    - dfxm12 12y ago
      What's not fair about getting a speeding ticket when you are driving over the speed limit?
      - lfuller 12y ago
        I believe it's implied that the limit itself is unfair.
      - Dylan16807 12y ago
        A largely-randomized system where most participants aren't punished but some are singled out for large fines is pretty unfair. Performing a bad action does not cause someone to forfeit all arguments as to fairness.
  - dfxm12 12y ago
    Where are these slower roads though? If I were a cop, I'd be much more likely to pull people over for speeding in more residential areas, which tend to have these sub-55 speed limits.
    - DSMan195276 12y ago
      I agree. Going 10 over on the freeway/highway hardly matters for the most part because it's not like that 10 mph is the difference between being able to or not able to stop if something runs out in-front of your car, you're already going at least 65 mph anyway. It's a different situation if it's a smaller road or a side-street with lots of houses around. Going 25 vs. 35 could very well be the difference between being able to stop fast enough if a kid runs out on the road.
- JamesSwift 12y ago
  To elaborate on why it "isn't worth it for the cops"... friends who are in law enforcement have explained to me that it's too easy to get a ticket thrown out for anything under 10mph due to arguments of device/speedometer precision.
- bluecalm 12y ago
  Most (all?) cars have speedometers which already overstate speed of a vehicle. This is I believe because of laws which forbid speedometer to overstate shown speed (by some margin) but not understate it so if you are driving 8.5mph above the limit it's probably actually even less than that.
- chris11 12y ago
  I think it also depends on the area. I did get pulled for 3 or 4 over the speed limit once. I think it was mainly because I was on a military base. Fortunately I did not actually get ticketed.
colanderman 12y ago
It's not an arbitrary premise, it's studied: “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.”
- astral303 12y ago
  Thank you for the scientific backup.
- _archon_ 12y ago
  Yes, according to that article. What I'm driving at is that making an arbitrary change based on how drivers react to an improper restriction may in fact lead to imposing another improper and arbitrary restriction. A change of 5 mph either way won't change anything fundamental, and so a traffic study will report minimal changes.
- timbre 12y ago
  This is consistent with most drivers behaving like _archon_ (and like me). For example, if every driver thinks she can safely drive 90 mph, but 80%, like _archon_ and me, stay below 10 over the limit out of fear of police, while 20% drive as fast as they like, then raising the limit from 60 to 70 will have no effect on the 85th percentile. The article seems to quietly conflate the fastest 15% of drivers ignoring the limit with "most drivers" ignoring the limit. Which is odd. However, even under the _archon_ model, increasing the limit would lead to a reduction in speed variance.
- streptomycin 12y ago
  And it's apparently controversial: Just as Megge can point to the results on hundreds or thousands of roads which have become more safe or equally safe when the speed limit increased, other researchers looking at data sets of speed limit changes have come to the opposite conclusion and advise that raising speed limits comes with the price of thousands of roadway fatalities. Although the article seems to support Megge and the studies he likes, it's not clear why (other than maybe that it makes for a more interesting blog post). And honestly, I can't imagine I'm the only one who feels comfortable going 80 on a 65 MPH highway but not on a 55 MPH highway. I guess it is possible, but it seems hard to believe.
mikeash 12y ago
Here's some support from a different direction. Starting a couple of years ago, Virginia started bumping up the speed limit on the more rural sections of interstates from 65 to 70MPH. Part of the stated rationale was that a lot of people speed anyway, but a lot of other people drive the speed limit, and this disparity is dangerous. By bumping up the limit, you're in essence telling the law-abiding drivers "it's OK to keep up with traffic" and the gap in speeds will narrow. In my experience, the opposite happened. It seems that the people who were driving 65 before don't want to go any faster. The people who were speeding before still constrained their speed to some factor above the speed limit, and now they go even faster still. Previously, the spread was around 65-75, now it's more like 65-80. In contrast, the interstates around DC are all limited to 55, yet people regularly do 70 there as well. It seems clear to me that, while speed limits don't impose an iron control on vehicle speeds, they do influence it. A sibling poster mentions studies that say it changes little if at all. Perhaps what's "clear" to me isn't true. However, I wonder just how those studies were performed. How long did they wait after the changed limit to measure the change in speed? Most traffic on most roads consists of people who make the trip regularly, and it will take time for them to change their habits, up or down.

mcdougle 12y ago

You do raise a good point. I know that, when I started driving, my dad taught me a neat trick: drive 8.5 mph over the speed limit, and you'll never get pulled over. That's (supposedly) because a ticket for anything less than 10mph is simply not worth it for the cops. If you go any more than 8.5mph over, though, you might accidentally cross that 10mph mark, and if a cop's around he'll get you. And, sure enough, any ticket I've ever gotten is because I was going over 10mph faster than the speed limit.

incision 12y ago
>'That's (supposedly) because a ticket for anything less than 10mph is simply not worth it for the cops.' Depends on your cops. I've witnessed tickets for 5 over (40 in a 45), 4 over (49 in a 45) and a stop, thankfully without a ticket for 1 over (31 in 30). I'm given to thinking of it as a lottery. Someone is getting pulled over, today me, tomorrow someone else. I'm just glad I haven't had a case of 'marijuana smell' for 10 years or so.
- mnarayan01 12y ago
  I think it more depends on your jurisdiction. When I'm driving somewhere new, I'll make sure I'm not going faster than average (even if that's below the speed limit). In places that I'm more familiar with, I'll do 9 over.
- mwfunk 12y ago
  I'm also in the "drive a little under 10 over" camp, and I totally accept that once every 10 years (or whenever), I'm going to be in the wrong place at the wrong time and get a ticket for it. I'm OK with that. It's not fair, but it's well below the bar of meaningful injustice for me to really care about it all that much.
  - dfxm12 12y ago
    What's not fair about getting a speeding ticket when you are driving over the speed limit?
    - lfuller 12y ago
      I believe it's implied that the limit itself is unfair.
    - Dylan16807 12y ago
      A largely-randomized system where most participants aren't punished but some are singled out for large fines is pretty unfair. Performing a bad action does not cause someone to forfeit all arguments as to fairness.
- dfxm12 12y ago
  Where are these slower roads though? If I were a cop, I'd be much more likely to pull people over for speeding in more residential areas, which tend to have these sub-55 speed limits.
  - DSMan195276 12y ago
    I agree. Going 10 over on the freeway/highway hardly matters for the most part because it's not like that 10 mph is the difference between being able to or not able to stop if something runs out in-front of your car, you're already going at least 65 mph anyway. It's a different situation if it's a smaller road or a side-street with lots of houses around. Going 25 vs. 35 could very well be the difference between being able to stop fast enough if a kid runs out on the road.
JamesSwift 12y ago
To elaborate on why it "isn't worth it for the cops"... friends who are in law enforcement have explained to me that it's too easy to get a ticket thrown out for anything under 10mph due to arguments of device/speedometer precision.
bluecalm 12y ago
Most (all?) cars have speedometers which already overstate speed of a vehicle. This is I believe because of laws which forbid speedometer to overstate shown speed (by some margin) but not understate it so if you are driving 8.5mph above the limit it's probably actually even less than that.
chris11 12y ago
I think it also depends on the area. I did get pulled for 3 or 4 over the speed limit once. I think it was mainly because I was on a military base. Fortunately I did not actually get ticketed.

colanderman 12y ago

It's not an arbitrary premise, it's studied: “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.”

astral303 12y ago
Thank you for the scientific backup.
_archon_ 12y ago
Yes, according to that article. What I'm driving at is that making an arbitrary change based on how drivers react to an improper restriction may in fact lead to imposing another improper and arbitrary restriction. A change of 5 mph either way won't change anything fundamental, and so a traffic study will report minimal changes.
timbre 12y ago
This is consistent with most drivers behaving like _archon_ (and like me). For example, if every driver thinks she can safely drive 90 mph, but 80%, like _archon_ and me, stay below 10 over the limit out of fear of police, while 20% drive as fast as they like, then raising the limit from 60 to 70 will have no effect on the 85th percentile. The article seems to quietly conflate the fastest 15% of drivers ignoring the limit with "most drivers" ignoring the limit. Which is odd. However, even under the _archon_ model, increasing the limit would lead to a reduction in speed variance.
streptomycin 12y ago
And it's apparently controversial: Just as Megge can point to the results on hundreds or thousands of roads which have become more safe or equally safe when the speed limit increased, other researchers looking at data sets of speed limit changes have come to the opposite conclusion and advise that raising speed limits comes with the price of thousands of roadway fatalities. Although the article seems to support Megge and the studies he likes, it's not clear why (other than maybe that it makes for a more interesting blog post). And honestly, I can't imagine I'm the only one who feels comfortable going 80 on a 65 MPH highway but not on a 55 MPH highway. I guess it is possible, but it seems hard to believe.

mikeash 12y ago

Here's some support from a different direction. Starting a couple of years ago, Virginia started bumping up the speed limit on the more rural sections of interstates from 65 to 70MPH. Part of the stated rationale was that a lot of people speed anyway, but a lot of other people drive the speed limit, and this disparity is dangerous. By bumping up the limit, you're in essence telling the law-abiding drivers "it's OK to keep up with traffic" and the gap in speeds will narrow. In my experience, the opposite happened. It seems that the people who were driving 65 before don't want to go any faster. The people who were speeding before still constrained their speed to some factor above the speed limit, and now they go even faster still. Previously, the spread was around 65-75, now it's more like 65-80. In contrast, the interstates around DC are all limited to 55, yet people regularly do 70 there as well. It seems clear to me that, while speed limits don't impose an iron control on vehicle speeds, they do influence it. A sibling poster mentions studies that say it changes little if at all. Perhaps what's "clear" to me isn't true. However, I wonder just how those studies were performed. How long did they wait after the changed limit to measure the change in speed? Most traffic on most roads consists of people who make the trip regularly, and it will take time for them to change their habits, up or down.