Following distance between cars

[Loren Booda]

"Following distance" between cars

What is an effective rule-of-thumb for determining the following distance between cars? It might incorporate reaction time; relation between velocity, energy and friction; brake/tire/road conditions; and behavior of other driver.

 

[Danger]

The generally accepted standard is the 2-second rule; ie: your vehicle should pass a given fixed point no less than 2 seconds after the preceding vehicle. I usually give it 3 seconds. Using time separation rather than car-lengths automatically adjusts for your speed.

 

[Ranger Mike]

3 second rule is best - at 60 MPH a car travels 88 feet per second. it takes 120 to 180 feet to bring a car to a compete stop ( depending upon the car, tires, brakes , pavement etc...)
tests show the average driver will have a 1 second reaction time once a hazard is realized.
now i know of no one who can judge 180 feet distance accurately while driving 60 MPH so the three second rule works for me..

 

[xxChrisxx]

2 seconds in the dry, 4 in the wet.

You just can't do that on modern roads though, if you leave a 2 second gap you get cut up like crazy.

I hate town driving and town drivers.

 

[daveg360]

The situation is crazy in the UK. Drivers are forced to learn an outdated table of "stopping distances" These were made in the late 60's in a Ford Anglia. Cars/tyres have progressed so much since that these values are so far from the truth that people make no correlation between them and real driving. On top of which they're listed in meters - who looks down the road and say "hmmm yes better give it 73m now I'm doing 60mph." The method discussed above using road side objects to count seconds is much more useful.

link to the stopping distances:
http://www.direct.gov.uk/prod_consum_dg/groups/dg_digitalassets/@dg/@en/documents/digitalasset/dg_070645.pdf

 

[Lojzek]

Probably the safety distance propotional to the velocity (constant safety time) is most reasonable. If this time is higher than your reaction time (usually about 1 s), then this will allow you to start braking before the car in front, so you can't crash if both can the same deceleration (brakes,tyres,braking skill).
Note that you could still crash if the car in front would stop instantly (for example crashing into a truck). Also you could crash if the car in front would have much better tyres, ABS or you panicked and pressed the brake too hard on the wet/icy road. A complete safety would be achieved with safety distance equal to your reaction+braking distance, which is a quadratic function of velocity: s=v*t+v^2/2a,
where t is reaction time, a is deceleration and v velocity.

One very dangerous situation is driving behind a speeder who shifts drive ways, because you can suddenly find yourself speeding towards a stopped vehicle: in this case constant safety time would not be enough to prevent accident.

 

[dlgoff]

I took a driver safety test not to long ago and 3 seconds was the rule and you would add 1 second for each situation; cloudy, wet, other drivers performance (weaving, etc.) ...

 

[Loren Booda]

What about icy conditions?

 

[dlgoff]

Cloudy, wet, and icy; add 3 seconds.

 

[russ_watters]

I don't consider the general rules of thumb reasonable. The need for you to leave the full distance required to stop assumes the car in front of you can stop instantly, in violation of Newton's laws*.

To me, minimum following distance is something you can't calculate in real life, but is figured intuitively based on the reaction time and deceleration time. Ie, assuming the total time from when the guy in front of you applies his brakes to when you do is a conservative 0.5 seconds and both of you accelerate from 60-0 in 3 seconds, you will close at 10 mph for 3 seconds, covering 44 feet.

*What this criteria does allow for is the car in front of you swerving to avoid an object in the road.

 

[Ranger Mike]

Russ
question - do you mean decelerate 60-0 mph and stop instantly is not possible unless a brick wall appears ..still not possible as a few seconds are required to move what's left of your body into the radiator portion of the car followed by the spare tire...

driving is survival , plain ans simple... any sane person would think it insane to fly across pavement at 88 feet per second in a rickety contraption...if we ever stopped long enuff to ponder what is actually happening..

I think rule of thumb is perfectly acceptable from this survival mode...especially when the clowns in the other cars are texting, talking on the cell phone or gulping down a cheese burger and never thinking about closing speed or clearance distance...yipes...

 

[Blenton]

On top of which they're listed in meters - who looks down the road and say "hmmm yes better give it 73m now I'm doing 60mph."

Don't people in the UK use Km/h to measure speed?

 

[Hurkyl]

I don't consider the general rules of thumb reasonable ... minimum following distance is something you can't calculate in real life

These are somewhat contradictory statements.

but is figured intuitively based on the reaction time and deceleration time.

At the very least, intuition requires experience. People get lots of experience driving when everything goes right -- but very little when things go wrong.

Ie, assuming the total time from when the guy in front of you applies his brakes to when you do is a conservative 0.5 seconds and both of you accelerate from 60-0 in 3 seconds, you will close at 10 mph for 3 seconds, covering 44 feet.

There are bad assumptions in this calculation. Even if we go with the 0.5 figure you suggest (despite others quoting numbers on the order of 1 second), this calculation also assumes that you slam your brakes as hard as you can every time the brake lights in front of you flash.

People don't drive like that.

 

[minger]

driving is survival , plain ans simple... any sane person would think it insane to fly across pavement at 88 feet per second in a rickety contraption...if we ever stopped long enuff to ponder what is actually happening..

As opposed to hurtling through the air at nearly 1000 ft/s in a rickety contraption using a principle that's not fully understood.

 

[Hurkyl]

And there are factors involved beyond a mere "if the guy in front of me stops, can I avoid hitting him?" For example:

. If someone pulls in front of me, do I still have any useful following distance?
. Can I maintain the heightened state of attention needed to react 'instantaneously'?
. Do I have enough room so that I can accelerate/decelerate smoothly as needed? (e.g. so that I don't have to slam the brakes every time the guy in front of me flashes his lights -- and that I don't have to worry as much about getting rear-ended by that guy riding my bumper)
. Is there room for someone in a neighboring lane to get over in case they have an emergency to deal with?

 

[russ_watters]

Russ
question - do you mean decelerate 60-0 mph and stop instantly is not possible unless a brick wall appears ..still not possible as a few seconds are required to move what's left of your body into the radiator portion of the car followed by the spare tire...

Huh? I didn't mention any brick walls. But it looks to me like the logic of following distance people use assumes something like having a brick wall appear in front of them, out of nowhere - and having to stop before they hit it.

driving is survival , plain ans simple... any sane person would think it insane to fly across pavement at 88 feet per second in a rickety contraption...if we ever stopped long enuff to ponder what is actually happening..

I don't consider it insane and don't see why you do - could you explain?

I think rule of thumb is perfectly acceptable from this survival mode...especially when the clowns in the other cars are texting, talking on the cell phone or gulping down a cheese burger and never thinking about closing speed or clearance distance...yipes...

If it is so acceptable, why does no one use it?

 

[russ_watters]

These are somewhat contradictory statements.

I'm not sure what you mean, but I'll explain further. I base my following distance on experience built from knowing how fast I can stop if the person in front of me slams on the brakes. Then, in the post, I analyzed that situation mathematically.

At the very least, intuition requires experience.

Yes.

People get lots of experience driving when everything goes right -- but very little when things go wrong.

I'm not sure what you mean by that, but I drive in traffic every day and see situations where the driver in front of me slams on their brakes every day.

There are bad assumptions in this calculation. Even if we go with the 0.5 figure you suggest (despite others quoting numbers on the order of 1 second), this calculation also assumes that you slam your brakes as hard as you can every time the brake lights in front of you flash.

I'd like to see some citations of that 1 second. It seems unreasonably high to me and as far as I'm concerned, what's off the top of my head seems more reasonable to me than what is off the top of theirs.

Note, though, that I didn't look at dave's link - it uses the same method I do! However, it uses 0.675 seconds as a reaction time. I'm fine with that being a typical reaction time. Now the reaction time of an athletic 33 year old would tend to be lower than typical, I'd thnk...

Dave's link also uses 0.67g as the deceleration, while I used close to 1, which in hindsight is probably unreasonably high.

People don't drive like that.

How do they drive? Based on the 3 second rule? As others noted, the 3 second rule is rarely followed. IMO, my methodology (quibble about my assumptions aside) is a reality about how people determine stopping distance, not some silly rule the DOT gave us that no one ueses.

 

[russ_watters]

If someone pulls in front of me, do I still have any useful following distance?

You cannot calculate the following distance behind an object that doesn't exist, so there is nothing you can do about that one.

Can I maintain the heightened state of attention needed to react 'instantaneously'?

Agreed - driver attentiveness affects following distance and one should allow more following distance when they know they are less attentive.

Do I have enough room so that I can accelerate/decelerate smoothly as needed? (e.g. so that I don't have to slam the brakes every time the guy in front of me flashes his lights -- and that I don't have to worry as much about getting rear-ended by that guy riding my bumper)

Agreed - I use more following distance when another person is in the car for their comfort and I use more following distance if someone is tailgating me.

Is there room for someone in a neighboring lane to get over in case they have an emergency to deal with?

This one I don't understand - how does following distance matter if you get sideswiped? You're not suggesting you apply a following distance rule to people next to you, are you?

 

[Ranger Mike]

Russ,,,,great posts...
i was just jerking everyones chain as i think everyone on this thread knows the dangers of cell phones in autos these days...you got to drive a car like riding a motorcycle..everyone is out to get you ...but i like the logic and discussion that takes place on this mech eng. formn..

 

[Hurkyl]

I'm not sure what you mean by that, but I drive in traffic every day and see situations where the driver in front of me slams on their brakes every day.

So that's the norm for you. Would you avoid a crash, though, if that driver slammed onto his brakes too late and runs into a "brick wall"? What about what that driver slams his brakes shortly after someone pulled in between you and him?

I'd like to see some citations of that 1 second. It seems unreasonably high to me and as far as I'm concerned,

It would be nice to see them on half-a-second too.

If all you're timing is the response time to seeing a car's brake lights turn on, then yah, I agree that one second is ridiculously high. But the number that matters has to account for the time it takes to realize you have to slam the brakes rather than gently press them, along with the maximum amount of distraction you might suffer (checking mirrors, reading street signs, and non-driving-related ones too).

How do they drive?

When they see brake lights in front of them, they don't slam the brakes on reaction. I can't tell how hard they do press at first, but it's definitely not a slam.

Based on the 3 second rule? As others noted, the 3 second rule is rarely followed ... a reality about how people determine stopping distance, not some silly rule the DOT gave us that no one ueses.

You should know better than to make an argumentum ad populum. Just because most people tailgate doesn't mean tailgating is safe.

You cannot calculate the following distance behind an object that doesn't exist, so there is nothing you can do about that one.

Sure I can, it's not hard. If I'm going 55 MPH down the highway following at half-a-second behind the guy in front of me, I can estimate that someone pulling between us will be roughly in the middle, and my following distance would be less than a sixth of a second.

The premise of your objection is flawed, though -- people really do pull in front of you. Therefore, safe driving must account for the fact it happens.

This one I don't understand - how does following distance matter if you get sideswiped?

This one was more about the safety of others. However, as an aside...

You're not suggesting you apply a following distance rule to people next to you, are you?

One of the usual guidelines of safe driving is not to drive side-by-side with other cars. (or just in front of or just behind)