Research reveals slow left lane drivers are spiking accident rates

[Duke87]

And it is well known that throughput and speed have very indirect, mostly inverse correlation. This is a type of correlation you would expect:

Throughput, as in vehicles per hour (per lane) is driven not by speed, but by intervals between cars - textbook 2 second interval tend to increase  at higher speed, and 1800 vph - that means 1 car every 2 second - is about peak value for non-congested traffic.

Several things here.

Firstly, yes, it is driven by intervals between cars. Intervals which can become larger if a left lane hog prevents other drivers from being able to access the space in front of him.

Secondly, someone driving unduly slowly and not allowing others to pass can help tip things into the bottom half of that graph where throughput decreases with speed (i.e. the congestion half).

Thirdly, even if we assume it has no effect on throughput, it still needlessly slows others down and prods them to do unsafe things.

Fourthly, avoiding unnecessarily being an obstacle to others is basic courtesy. If you're walking slowly and someone says "excuse me", you don't stubbornly continue obstructing them, you step aside and let them pass. Other drivers on the road cannot verbally communicate "excuse me" but the same basic principle otherwise still applies.


As for the issue of people merging in too closely in front of other vehicles, that is also bad and dangerous behavior. But that doesn't mean the alternative is to just stay in the left lane, the proper alternative is to move right when you have sufficient space to safely do so.

[Sykotyk]

Looks like another certified traffic engineer with bogus pics. You realize that red car goes 3x faster than the blue one? Actually, I don't think there is any situation when 3x speed difference may be legitimate unless blue car is broken. Most likely red driver is speeding beyond any reason. No, we don't need to increase throughput at that cost.
Neither we need drivers like green one on the right pic, cutting blue guy with less than 1/4 second interval. 
Don't behave like you're running for political office, lie within the reason!

The GIF is designed to provide a basic visual demonstration of the effects of camping in the left lane, not to provide a perfect to scale movie of the situation.

The differences in speed may be exaggerated, and the green car may be shown merging what appears to be very closely in front of the blue car, but this doesn't change the underlying principle. Make a GIF with these details corrected and the red car would still move out of the frame sooner in the scenario where the green car gets in the right lane.

Where that difference really presents itself is when the green vehicle does get over, but has to wait for the red car to clear first to allow the cars behind red to then pass green on the left.

[kalvado]

Several things here.

Firstly, yes, it is driven by intervals between cars. Intervals which can become larger if a left lane hog prevents other drivers from being able to access the space in front of him.

Secondly, someone driving unduly slowly and not allowing others to pass can help tip things into the bottom half of that graph where throughput decreases with speed (i.e. the congestion half).

less obvious is that deviating too far from average in either way can cause problems.

Thirdly, even if we assume it has no effect on throughput, it still needlessly slows others down and prods them to do unsafe things.

Often needlessly slows down to as slow as speed limit+20!

Fourthly, avoiding unnecessarily being an obstacle to others is basic courtesy. If you're walking slowly and someone says "excuse me", you don't stubbornly continue obstructing them, you step aside and let them pass. Other drivers on the road cannot verbally communicate "excuse me" but the same basic principle otherwise still applies.

driving slalom equates "excuse me" with a bang of body hitting the wall next millisecond.

Once again - at higher vehicle density creating and maintaining a dual hump distribution may require slowing higher speed part of distribution, accelerating lower speed part of distribution (well, IF possible - as mentioned above, heavy truck in the mountains is physically unable to do that; but accelerating it will improve things) and sorting traffic between lanes.

As for the issue of people merging in too closely in front of other vehicles, that is also bad and dangerous behavior. But that doesn't mean the alternative is to just stay in the left lane, the proper alternative is to move right when you have sufficient space to safely do so.

IF, not WHEN.  At certain traffic density that becomes a critical difference.

[kalvado]

Just found an interesting summary of state laws, looks like there are several approaches:
1. Keep right except to pass: IL, IN, LA, MA, NJ, many roads posted as such in TX - 5.5 total; CO, KY, ME - same at 65 MPH or higher limit (3 more)
2. slower traffic keep right (IMHO most reasonable approach): AL, AZ, CA, CO, DE, HI, ID, IA, MN, MS, MI, MT, NE, NH, NM, NY, ND, OK, OR, RI, SC, TN, TX, VT, WV, WY; CT - same AND no passing in right lane with 2 travel lanes in same direction (???) ]\-  seem most common
3. Yield to faster traffic (I am not sure where it land between 1 and 2): FL, GA, UT, VA  - 4  (TN also moves here in july)
4. cannot go below speed limit in left: AK, NC, OH, PR (I wonder how that works is congested or stop-and-go traffic) - 4 total
5. SD, NV: SLOW (not slower!) vehicles need to keep right.  NV has strange wording, somewhat closer to "yield" - 2 total

Rules are a often bit fuzzy, WA for example is "except to pass" - but exemption is for vehicles "moving faster than traffic". IL - has strict wordings, but left lane hogging is OK if there is now vehicle directly behind.. does that amount to "yield"?

[RobbieL2415]

Just found an interesting summary of state laws, looks like there are several approaches:
1. Keep right except to pass: IL, IN, LA, MA, NJ, many roads posted as such in TX - 5.5 total; CO, KY, ME - same at 65 MPH or higher limit (3 more)
2. slower traffic keep right (IMHO most reasonable approach): AL, AZ, CA, CO, DE, HI, ID, IA, MN, MS, MI, MT, NE, NH, NM, NY, ND, OK, OR, RI, SC, TN, TX, VT, WV, WY; CT - same AND no passing in right lane with 2 travel lanes in same direction (???) ]\-  seem most common
3. Yield to faster traffic (I am not sure where it land between 1 and 2): FL, GA, UT, VA  - 4  (TN also moves here in july)
4. cannot go below speed limit in left: AK, NC, OH, PR (I wonder how that works is congested or stop-and-go traffic) - 4 total
5. SD, NV: SLOW (not slower!) vehicles need to keep right.  NV has strange wording, somewhat closer to "yield" - 2 total

Rules are a often bit fuzzy, WA for example is "except to pass" - but exemption is for vehicles "moving faster than traffic". IL - has strict wordings, but left lane hogging is OK if there is now vehicle directly behind.. does that amount to "yield"?

How many states have exceptions for roads with three or more lanes?

[jeffandnicole]

4. cannot go below speed limit in left: AK, NC, OH, PR (I wonder how that works is congested or stop-and-go traffic) - 4 total

LOL.  I'd love to see the actual laws in those states.  I'm quite sure the law isn't "You must illegally speed to use the left lane".

That's why those summaries, probably found on some quasi-clickbait website, aren't what should be used as guides to anything.

[kalvado]

4. cannot go below speed limit in left: AK, NC, OH, PR (I wonder how that works is congested or stop-and-go traffic) - 4 total

LOL.  I'd love to see the actual laws in those states.  I'm quite sure the law isn't "You must illegally speed to use the left lane".

That's why those summaries, probably found on some quasi-clickbait website, aren't what should be used as guides to anything.

AK     
13 AAC 002.50(b):

Upon all roadways outside an urban district, a vehicle other than an emergency vehicle proceeding at less than the maximum authorized speed of traffic must be driven in the right-hand lane or as close as practicable to the right-hand curb or edge of the roadway, except when ....

NC     

20-146(b):

(b)        Upon all highways any vehicle proceeding at less than the legal maximum speed limit shall be driven in the right-hand lane then available for thru traffic, or as close as practicable to the right-hand curb or edge of the highway, except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn.
...

(e)        Notwithstanding any other provisions of this section, when appropriate signs have been posted, it shall be unlawful for any person to operate a motor vehicle over and upon the inside lane, next to the median of any dual-lane highway at a speed less than the posted speed limit when the operation of said motor vehicle over and upon said inside lane shall impede the steady flow of traffic

[jeffandnicole]

4. cannot go below speed limit in left: AK, NC, OH, PR (I wonder how that works is congested or stop-and-go traffic) - 4 total

LOL.  I'd love to see the actual laws in those states.  I'm quite sure the law isn't "You must illegally speed to use the left lane".

That's why those summaries, probably found on some quasi-clickbait website, aren't what should be used as guides to anything.

AK     
13 AAC 002.50(b):

Upon all roadways outside an urban district, a vehicle other than an emergency vehicle proceeding at less than the maximum authorized speed of traffic must be driven in the right-hand lane or as close as practicable to the right-hand curb or edge of the roadway, except when ....

NC     

20-146(b):

(b)        Upon all highways any vehicle proceeding at less than the legal maximum speed limit shall be driven in the right-hand lane then available for thru traffic, or as close as practicable to the right-hand curb or edge of the highway, except when overtaking and passing another vehicle proceeding in the same direction or when preparing for a left turn.
...

(e)        Notwithstanding any other provisions of this section, when appropriate signs have been posted, it shall be unlawful for any person to operate a motor vehicle over and upon the inside lane, next to the median of any dual-lane highway at a speed less than the posted speed limit when the operation of said motor vehicle over and upon said inside lane shall impede the steady flow of traffic

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

[kalvado]

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

[bzakharin]

So it's saying "keep right except to pass or while speeding", though I'm sure there are also "no speeding" laws, so basically, just like the other states.

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

I'm pretty sure it's written into the laws that you can't be pulled over for violating min or max speed laws by less than 5 MPH due to acceptable inaccuracies in speedometers

[kalvado]

So it's saying "keep right except to pass or while speeding", though I'm sure there are also "no speeding" laws, so basically, just like the other states.

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

I'm pretty sure it's written into the laws that you can't be pulled over for violating min or max speed laws by less than 5 MPH due to acceptable inaccuracies in speedometers

NYS specifically stresses that 1 MPH over is already punishable.

[jeffandnicole]

So it's saying "keep right except to pass or while speeding", though I'm sure there are also "no speeding" laws, so basically, just like the other states.

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

I'm pretty sure it's written into the laws that you can't be pulled over for violating min or max speed laws by less than 5 MPH due to acceptable inaccuracies in speedometers

The tolerance for your "speedometer being out of whack" is a myth.  You're supposed to have a properly maintained car.  Doesn't matter what's wrong with it...that's not the cop's fault.  If you have 3 brake lights but 1 is out, there's no 'tolerance' just because the other 2 are working.  In PA they give you a 6 mph leeway if using radar in normal zones..  But via other means of enforcement, or in work zones, they can ticket for 1 over.  Again, has nothing to do with your speedometer...it has to do with their equipment.

[kphoger]

As for the issue of people merging in too closely in front of other vehicles, that is also bad and dangerous behavior. But that doesn't mean the alternative is to just stay in the left lane, the proper alternative is to move right when you have sufficient space to safely do so.

IF, not WHEN.  At certain traffic density that becomes a critical difference.

In congested traffic, (a) faster traffic is generally, in my experience, more willing to wait and less likely to engage in dangerous maneuvers; and (b) exceptions are generally written into the traffic laws.  As examples of the latter, the state of Illinois specifically excludes "when traffic conditions and congestion make it impractical to drive in the right lane" from the regulation; my current home state of Kansas does not, but rather the law is restricted to highways "located outside the corporate limits of any city", which is obviously based on the same intent.  When traffic is congested, in fact, it's been shown that changing lanes as little as possible is actually the best way to increase throughput; this is the reason converting one lane into an HOV lane can actually improve throughput, because lane changes are less frequent with fewer opportunities to do so.

The problem, in my experience (hardly scientific), is when drivers avoid moving right into a large-enough space because doing so would require them to slow down by 3 mph for a minute or two–instead irritating all the drivers behind them who are traveling at a higher speed.  That's the sort of behavior that encourages people to cut off the right-lane vehicle, then gun it in the right lane and squeeze ahead of the left lane dawdler.  If there is room enough in the right lane for the daredevil to complete that maneuver, then there was room enough for the dawdler to have moved right and "hung out" for a minute while faster traffic passed by.

As I see it, the actual speed of the vehicles in question compared to the posted speed limit shouldn't factor into the conversation.  All vehicles could be going 10 mph under the limit and slower, all vehicles could be going 10 mph over the limit and faster, or it could be a mix.  But slower traffic keeping right is both common courtesy on the road (almost universally) and a rule of the road (in an increasing number of jurisdictions) independent of the speed limit.  Obstructing the free flow of traffic is both dangerous and illegal, no matter what absolute speed the vehicles are actually going.

[jeffandnicole]

If there is room enough in the right lane for the daredevil to complete that maneuver, then there was room enough for the dawdler to have moved right and "hung out" for a minute while faster traffic passed by.

+Infinity

[kalvado]

As for the issue of people merging in too closely in front of other vehicles, that is also bad and dangerous behavior. But that doesn't mean the alternative is to just stay in the left lane, the proper alternative is to move right when you have sufficient space to safely do so.

IF, not WHEN.  At certain traffic density that becomes a critical difference.

In congested traffic, (a) faster traffic is generally, in my experience, more willing to wait and less likely to engage in dangerous maneuvers; and (b) exceptions are generally written into the traffic laws.  As examples of the latter, the state of Illinois specifically excludes "when traffic conditions and congestion make it impractical to drive in the right lane" from the regulation; my current home state of Kansas does not, but rather the law is restricted to highways "located outside the corporate limits of any city", which is obviously based on the same intent.  When traffic is congested, in fact, it's been shown that changing lanes as little as possible is actually the best way to increase throughput; this is the reason converting one lane into an HOV lane can actually improve throughput, because lane changes are less frequent with fewer opportunities to do so.

The problem, in my experience (hardly scientific), is when drivers avoid moving right into a large-enough space because doing so would require them to slow down by 3 mph for a minute or two–instead irritating all the drivers behind them who are traveling at a higher speed.  That's the sort of behavior that encourages people to cut off the right-lane vehicle, then gun it in the right lane and squeeze ahead of the left lane dawdler.  If there is room enough in the right lane for the daredevil to complete that maneuver, then there was room enough for the dawdler to have moved right and "hung out" for a minute while faster traffic passed by.

As I see it, the actual speed of the vehicles in question compared to the posted speed limit shouldn't factor into the conversation.  All vehicles could be going 10 mph under the limit and slower, all vehicles could be going 10 mph over the limit and faster, or it could be a mix.  But slower traffic keeping right is both common courtesy on the road (almost universally) and a rule of the road (in an increasing number of jurisdictions) independent of the speed limit.  Obstructing the free flow of traffic is both dangerous and illegal, no matter what absolute speed the vehicles are actually going.

Another pet peeve of mine is shifting right, slowing down  - only to realize that the guy pressing me out of left lane now went to camp out at 7 o'clock blind spot position... But that is just me

Yes, basically goal - at moderate traffic densities, 600-900 vph per lane - to form two streams in 2 lanes, with 3-5-10 MPH speed differential. That requires, among other things, top 20% to slow down.  Comparing actual speed of those fast vehicles to speed limit (actually design speed or free flow speed, but those are somewhat correlated)  may be useful to understand if they belong to top 20%.

In fact, at moderate-higher traffic density, there will be 2 speeds to choose from: right lane speed (strongly affected by trucks), or left lane car-mostly speed. Don't be an asshole, do not hold left lane. (heavy truck may have no other option than holding right lane, though) Do not be an asshole, do not push everyone else into right lane so you can keep your TOP-10% rating.

[kalvado]

If there is room enough in the right lane for the daredevil to complete that maneuver, then there was room enough for the dawdler to have moved right and "hung out" for a minute while faster traffic passed by.

+Infinity

No. Daredevil may be comfortable with 10' between bumpers (they can do 5 easily). Good driver should remember 2 seconds intervals. 

[kphoger]

Good driver should remember 2 seconds intervals. 

Just pointing out that a 2-second interval is only a rule of thumb, and is not even advocated everywhere.  I recall hearing from someone that students in Germany are taught a 1-second or a 1.5-second interval, and scientific studies there have shown the average interval in practice ranges from roughly 1.2 seconds to 2 seconds, with trucks not even topping 3 seconds.

[edited to add "in Germany]

[jeffandnicole]

You mean AAA telling us the proper spacing should be 3 seconds isn't remotely attainable in even moderately light traffic?

[kalvado]

Good driver should remember 2 seconds intervals. 

Just pointing out that a 2-second interval is only a rule of thumb, and is not even advocated everywhere.  I recall hearing from someone that students in Germany are taught a 1-second or a 1.5-second interval, and scientific studies there have shown the average interval in practice ranges from roughly 1.2 seconds to 2 seconds, with trucks not even topping 3 seconds.

[edited to add "in Germany]

Doesn't look like german highways are any different from US:
http://iopscience.iop.org/article/10.1088/0305-4470/35/15/302/meta
German folks study traffic on german highways and see maximum throughput at 18 vehicles per km,  170' intervals - about 12 car lengths... 

[kphoger]

From an article I don't have to pay to read:

The drivers fall below the minimum distance of 0.9 seconds as required by German law in 41% of the total time. ... Mean values of the individual drivers are in the range between 0.7 and 1.6 seconds

[kalvado]

From an article I don't have to pay to read:

The drivers fall below the minimum distance of 0.9 seconds as required by German law in 41% of the total time. ... Mean values of the individual drivers are in the range between 0.7 and 1.6 seconds

No contradiction.. They are looking at high traffic densities, 40+ cars per km - which means beginning of congestion, reduction of speed  and skyrocketing lane changes. Of you will, their selection criteria is 60 km/h (38 MPH) at speed limits 100 and above (or no limit).   

[Pete from Boston]

Not to get all Yoda or anything, but...
reducing throughput leads to added congestion,
  added congestion leads to road rage,
   road rage leads to unsafe maneuvers,
    unsafe maneuvers lead to accidents.

On a public policy level, added congestion leads to road rage.

In reality, individuals' lack of control over their emotions leads to road rage.

I get your point, but I tire of the idea that because a lot of people lack minimal social skills, those individuals lack personal responsibility for what they do. 

I sometimes shop at an extremely congested supermarket (Somerville Market Basket, for locals familiar).  On weekends you stand and stand waiting just to move across the front of the store.  Sometimes its volume, sometimes it's one person's carelessness.

If someone yelled obscenities at slow people in the supermarket, no one would stand for it.  The yeller would be roundly told to be patient like everyone else.  But put them in a glass box on the road, and their behavior is suddenly explainable as being caused by those around them, as part of a social phenomenon.

[Jardine]

So what is the motive for the undesirable behavior of driving slow in the left lane, and how do you remove that motive?

In my experience it is a combination of laziness and obliviousness. Someone will get in the left lane to pass a couple trucks or whatever, and then just start daydreaming and cruising in the left lane rather than moving back right, neither noticing nor caring that people are passing them on the right.

In other words, hogging the left lane is not typically something which is done deliberately for any sort of selfish gain. It is something which is done unintentionally due to a driver's lack of situational awareness.

This is therefore something that perhaps a target enforcement campaign could be good medicine for. If a driver remembers that time they got a ticket for driving too slowly in the left lane, maybe they'll pay more attention next time.

In all my years driving, less than a handful of times has it been some deliberately slow in the left lane. One of the worst offenders was someone hellbent on keeping everyone behind them. They'd speed up until there was another car beside them on the right. Then slow to match their speed. They did this for miles and miles. Had a huge line of cars. Maybe some power-hungry cog at some factory wanted to feel important.

But, more often than not, it's obliviousness or just not caring. They get in the left lane to pass someone and just stay there. Even if it's clear other traffic has come up on them. They're not moving. A lot of cell-phone yakkers or texters, as well. I guess they think in the left lane they don't have to deal with on and off ramp traffic to mess with their clear view down the road.

LOL, I have been known to do the opposite.

If I note someone in the left lane going too slow, I'll align with their vehicle and then match their speed in the right lane and then watch the fun and festivities and mayhem in the left lane.

[bzakharin]

So it's saying "keep right except to pass or while speeding", though I'm sure there are also "no speeding" laws, so basically, just like the other states.

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

I'm pretty sure it's written into the laws that you can't be pulled over for violating min or max speed laws by less than 5 MPH due to acceptable inaccuracies in speedometers

The tolerance for your "speedometer being out of whack" is a myth.  You're supposed to have a properly maintained car.  Doesn't matter what's wrong with it...that's not the cop's fault.  If you have 3 brake lights but 1 is out, there's no 'tolerance' just because the other 2 are working.  In PA they give you a 6 mph leeway if using radar in normal zones..  But via other means of enforcement, or in work zones, they can ticket for 1 over.  Again, has nothing to do with your speedometer...it has to do with their equipment.

I found this (https://www.fmcsa.dot.gov/regulations/title49/section/393.82), applicable to buses and trucks:
"Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)."

Now if a speedometer is allowed by law to be off by 10% in either direction, then it follows that you cannot be held accountable for doing 55 in a 50 MPH zone or 71 in a 65 zone. And it hardly makes sense to have truck and bus regulations more lenient than car regulations

[cl94]

So it's saying "keep right except to pass or while speeding", though I'm sure there are also "no speeding" laws, so basically, just like the other states.

None of them said driving over the speed limit is legal in the left lane.  Driving at the posted speed limit (70 in a 70) isn't driving under the speed limit.

It is catch 22: you may be fined for 71 as well as for 69.And even cruise control may have some speed variations...  Makes road profiling really simple: anyone can be pulled over without a problem.

I'm pretty sure it's written into the laws that you can't be pulled over for violating min or max speed laws by less than 5 MPH due to acceptable inaccuracies in speedometers

The tolerance for your "speedometer being out of whack" is a myth.  You're supposed to have a properly maintained car.  Doesn't matter what's wrong with it...that's not the cop's fault.  If you have 3 brake lights but 1 is out, there's no 'tolerance' just because the other 2 are working.  In PA they give you a 6 mph leeway if using radar in normal zones..  But via other means of enforcement, or in work zones, they can ticket for 1 over.  Again, has nothing to do with your speedometer...it has to do with their equipment.

I found this (https://www.fmcsa.dot.gov/regulations/title49/section/393.82), applicable to buses and trucks:
"Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)."

Now if a speedometer is allowed by law to be off by 10% in either direction, then it follows that you cannot be held accountable for doing 55 in a 50 MPH zone or 71 in a 65 zone. And it hardly makes sense to have truck and bus regulations more lenient than car regulations

That and the issues with keeping radar guns perfectly calibrated are why cops typically don't pull people over if doing less than 7-8 above. Too easy to fight it.