Following Distances

[jakeroot]

I've seen this discussed various times on different threads, but I don't recall seeing a thread dedicated to the topic.

What kind of following distances do you generally leave during various conditions?

I generally leave a 1.5-2 seconds on the freeway (anywhere from 55 to 75 mph), though during very heavy traffic, this can drop to 1 second.

I'm taking this National Safety Council Defensive Driving Course, and it recommends a five second following distance during rush hour. WHAT??!!?!

The only reason I chose three over two was due the part before it having said three was the minimum.

[webny99]

I'm taking this National Safety Council Defensive Driving Course, and it recommends a five second following distance during rush hour. WHAT??!!?!

I'm not very structured with following distances; I'm probably a bit too intuitive and not observant enough in this regard. If it feels like I'm getting too close, I'll do one of the following: (1) back off without braking (2) pass if I can, or (3) do a left blinker if they can move right.

Interestingly, if I'm approaching visible congestion or a known congested area, I'll always increase following distance with an initial brake - and then try to maintain the new speed. In congestion, building in some distance allows for more consistent speed - to the point where you can (almost) completely eliminate the stop-and-go oscillations for everyone behind you. Braking in congestion contributes to the problem, and I try to avoid it at all costs.

[NoGoodNamesAvailable]

I leave 2 seconds minimum, 3 seconds if possible on dry pavement. This will lead to people pulling in front of you in heavy traffic on multilane roads, but this doesn't really bother me. In the end a few extra cars in front of me is not going to drastically affect my arrival time, and it's much less stressful to drive with a larger gap and creep instead of stopping and starting.

[hbelkins]

I was taught two seconds.

If anyone has a good photo of the minimum spacing dots that are posted on some Pennsylvania highways, this would be a good time to post it. I've seen these on I-80 in the eastern part of the state and also on US 322 outside State College.

[vdeane]

If anyone has a good photo of the minimum spacing dots that are posted on some Pennsylvania highways, this would be a good time to post it. I've seen these on I-80 in the eastern part of the state and also on US 322 outside State College.

http://nysroads.com/photos.php?route=pa147&state=PA&file=101_7992.JPG

[US 89]

In my driver's ed class, I was taught to leave 4 seconds following distance on freeways, and 2 seconds on other roads.

[1995hoo]

The problem around here, and probably in most urban areas, is that it's impossible to leave the "correct"  amount of space (regardless of whether you use seconds or carlengths) because people will see it as an invitation to cut in.

I don't follow any one rule of thumb on distance. Depends on weather, traffic, which car I'm driving (due to differences in brakes and weight), speed, visibility....lots of factors. I recall in driver's ed they taught two seconds under 40 mph and four seconds over 40. Four seconds is way too much.

My parents and my wife (she is slightly older than I am and grew up in a different part of the country) were taught carlengths; my wife just said she was taught one carlength per 10 mph and she then added, "But nobody follows that anymore."  (Partly for the same reason I noted about people cutting in, partly because now they teach seconds instead, which she finds bizarre and absurd.)

BTW, my wife's TLX has adaptive cruise control. I had to decrease the following distance. It left too much space.

[webny99]

The problem around here, and probably in most urban areas, is that it's impossible to leave the "correct"  amount of space (regardless of whether you use seconds or carlengths) because people will see it as an invitation to cut in.

In congested conditions, I'm willing to allow people to cut in front of me in exchange for my being able to maintain a consistent speed, thereby improving traffic flow.

Much less so in free-flowing conditions.

[Hurricane Rex]

Most people just do what's comfortable. For me 3 seconds in the city, and 4 seconds on Freeways.

[Jardine]

Much to the confusion and consternation of my fellow motorists, I try to leave extra space ahead of me when the roads are slick with ice.

Maybe not so bad here in Iowa, but when I tried doing it in Wisconsin the reactions ranged from abject bewilderment to overt hostility.

[jeffandnicole]

Another guide is to leave 1 car length for every 10 mph.  In the above example, the highlighted car is about 2.5 car lengths behind the truck, appropriate for a 25 mph speed limit. 

Being that the roadway doesn't appear to be a 25 mph roadway, the example provided doesn't even properly illustrate what a 5 second distance would look like.  Even worse, in the right lane, the car is only about 1.5 car lengths behind the truck.

In my daily rush hour commuting, you're lucky to have 1 second of space between vehicles at 70mph.  2 seconds invites people to merge between you.  You couldn't even get a 5 second gap if you wanted to.

[jakeroot]

In my daily rush hour commuting, you're lucky to have 1 second of space between vehicles at 70mph.  2 seconds invites people to merge between you.  You couldn't even get a 5 second gap if you wanted to.

That's my problem with suggested following distances. In practice, they're near impossible to follow. A five second gap at 30 mph is 220 feet, or about 3.5 semi trucks (back to back). That is a gigantic following distance for that speed, with the suggested distance only growing as speed grows. At 60 mph? About seven semi trucks (or 30 cars back to back!). That's laughable. Everyone is going to pull in front of you. You'll be miles behind where you could have been.

I wish the question told me how fast traffic was moving. Just because it's rush hour, doesn't mean we aren't going the speed limit.

In the UK, the suggested following distance is two seconds. The saying goes, "only a fool breaks the two second rule". Two seconds at 70 mph is about 204 feet, or less than 3.5 semi trucks. That seems pretty reasonable to me. Cars stop much shorter than they used to. Sure, there's still the "perception" and "reaction" distances, but the actual stopping distances have been shortened up quite a bit thanks to ABS. And yes, ABS reduces stopping distances overall because it takes the skill out of pumping brakes. Everyone can stop quickly now, versus only those who could remember to pump the brakes when it mattered.

[kalvado]

Car intervals have very direct relation to highway throughput.
2 second interval (plus a little bit for car length) corresponds to 1700-1800 vehicles per hour per lane.
5 second intervals reduce things to below  1000 VPH.  We need to double lane count for that.

[kkt]

Yes, five seconds is way longer than necessary and very difficult to maintain in rush-hour conditions even if you wanted to.  Try for three seconds.

[kphoger]

I generally leave a 1.5-2 seconds on the freeway (anywhere from 55 to 75 mph), though during very heavy traffic, this can drop to 1 second.

That approximates my driving practice as well.  And that of pretty much everyone else I see out there on the road.

Depends on weather, traffic, which car I'm driving (due to differences in brakes and weight), speed, visibility....lots of factors.

I'll throw another factor in to the list:  familiarity with the car.  Basically, I know how well my brakes respond to a sudden stop.  In someone else's car, not so much, so I leave a little more cushion than I would in my own car.

Much to the confusion and consternation of my fellow motorists, I try to leave extra space ahead of me when the roads are slick with ice.

Maybe not so bad here in Iowa, but when I tried doing it in Wisconsin the reactions ranged from abject bewilderment to overt hostility.

Good for you!  Ice gets reeeeeealy dicey.  There seem to be primarily two types of drivers in winter conditions:  the ones who think you have to go 20 mph, and the ones who think there's no problem doing their usual speed.  Those of us in between seem to be few and far between (see what I did there?)

[1995hoo]

Another guide is to leave 1 car length for every 10 mph.  ....

....

My parents and my wife (she is slightly older than I am and grew up in a different part of the country) were taught carlengths; my wife just said she was taught one carlength per 10 mph and she then added, "But nobody follows that anymore."  (Partly for the same reason I noted about people cutting in, partly because now they teach seconds instead, which she finds bizarre and absurd.)

....

....

Depends on weather, traffic, which car I'm driving (due to differences in brakes and weight), speed, visibility....lots of factors.

I'll throw another factor in to the list:  familiarity with the car.  Basically, I know how well my brakes respond to a sudden stop.  In someone else's car, not so much, so I leave a little more cushion than I would in my own car.

....

That's an excellent point.

[kalvado]

Another guide is to leave 1 car length for every 10 mph.

My car is about 190" long. At 60 MPH=88 fps, six car length is 95' or 1.08 seconds. Would be about true for any other speed.
Probably cars were longer back then, and the rule gave something closer to 1.5 seconds.
In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

[kphoger]

In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

You get really close, then have the passenger hook a tape measure onto the vehicle, then drop back until the tape measure reads an appropriate number, then get really close again to unhook the tape measure, then drop back again.

[kalvado]

In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

You get really close, then have the passenger hook a tape measure onto the vehicle, then drop back until the tape measure reads an appropriate number, then get really close again to unhook the tape measure, then drop back again.

Problem is... I am ashamed to say that... I am not sure about exact length of my car... I had to google that number.
Now feel free to expel me from the community, but I have to be honest!

[kphoger]

In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

You get really close, then have the passenger hook a tape measure onto the vehicle, then drop back until the tape measure reads an appropriate number, then get really close again to unhook the tape measure, then drop back again.

Problem is... I am ashamed to say that... I am not sure about exact length of my car... I had to google that number.
Now feel free to expel me from the community, but I have to be honest!

And they let you drive on the road!   

[slorydn1]

I usually follow the 2 second rule in normal traffic at cruising speed.

When things begin to tighten up, I may close in a little if I can see through the car in front of me. If I'm stuck behind a large SUV or truck I'll drop back a little further since all I can see is their rear bumper. I do, however, stand ready to close the gap at a moments notice if I feel like someone is thinking about butting in. I guess its the last vestige of my Chicagoland driving education that still remains.

[jakeroot]

I was driving in some quick-moving heavy traffic yesterday, and my following distance (along with others) was down to, "one mississi..." Might have been too close.

[kalvado]

I was driving in some quick-moving heavy traffic yesterday, and my following distance (along with others) was down to, "one mississi..." Might have been too close.

I hope that was not "one mississi... BOOM! OH SHIT!!!"

[kkt]

In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

You get really close, then have the passenger hook a tape measure onto the vehicle, then drop back until the tape measure reads an appropriate number, then get really close again to unhook the tape measure, then drop back again.

Photos or it didn't happen 

[kphoger]

In either case, I  don't quite understand how to measure distance to another car in car lengths. time is an easier metrics for me. 

You get really close, then have the passenger hook a tape measure onto the vehicle, then drop back until the tape measure reads an appropriate number, then get really close again to unhook the tape measure, then drop back again.

Photos or it didn't happen 

Right.  I forgot that step.