Exploring the Unsensical Rule of Thumb for Following Distance

[russ_watters]

It's wrong that a vehicle take more time to brake from 200-150 kmh than from 50-0 kmh given the same braking force appleid in both cases? Tht can't be since from 200-150 kmh much more KE has to be taken away as compared to 50-0 kmh.

The time is the same, the kinetic energy is different. acceleration=force/mass, but work=force*distance. Force is not the time change in kinetic energy of a given mass, it is the time change in momentum of a given mass.

Said another way, your brakes will get hotter when you decelerate from 200-150, than from 50-0, but it will take the same amount of time.

 

[russ_watters]

Hi "dave",

German lawmakers say keep a following distance = (.5 X speed in kmh) meters, meaning at 50 kmh you should have an fd of 25, at 100kmh an fd of 50m at 200kmh an fd of 100m and so on. This is what i called nonsense basically. Also read my post previous to that and then comment please.

German lawmakers are trying to avoid collisions, you are trying to lessen the severity of collisions. Which is the better criteria for deciding on following distance? I suppose it is partly a matter of opinion, but the reality is that more accidents will mean more injuries and more deaths because collisions aren't one dimensional. A small collision at highway speed turns into a big collision when the cars start spinning and hitting other cars, jumping the guard rails, etc.

The criteria the lawmakers use is better than the one you use.

 

[russ_watters]

But as i said keeping a greater distance btw the front car when BOTH you and the one infront are traveling faster won't be more effective in avoiding a collision either. The distance traveled within reaction time towards the back car by the front one does NOT increase if if it brakes and both travel faster BUT actually DECREASES because the front car is now slowing by the same amount of speed it wld have at lower speeds IN MORE TIME. So if anything keeping a greater distance while trvaelling at high speed is dangerous coz you might notice the car braking tooo late when it's already come 3m close to u. NOW how wld u avoid a collison. You most probanly can't n now the impact speed wld be heavy too. Got it?

That's just plain absurd/nonsensical and if you actually tried to apply some math and calculate what you are talking about (heck, I already did that calculation for you in post 7!) you would realize it.

Unless you are going to try to learn the physics behind what you are saying, there won't be much point in continuing this thread - you're just spouting nonsense here.

To sum up:
1. Lawmakers calculate appropriate following distance based on the desire to reduce the number of crashes.
2. Due to reaction time causing a speed differential, following distance must increase with speed in order to avoid collisions.

 

[Danger]

That's a rediculous amount of following distance and it would surprise me greatly if you actually left that much.

No offense, Russ, but if that's typical of US driver thinking, it's no wonder you guys keep getting into 20-car pile-ups in situations where anyone with an ounce of ability would escape safely.

 

[YellowTaxi]

Probably beside the point, but I'm not so sure a faster car decelerates any slower than a slower car. Perhaps if the brakes were inadequate, but otherwise, why would it?

Just to add to the confusion on here,
Higher wind resistance occurs at higher speeds. Thus a faster car does definitely deccelerate better than an identical car traveling more slowly. So I assume we should now also be driving as fast as possible on highways - and we can be safe in the knowledge that we can deccelrate better than anybody else on the same street. ?

 

[rcgldr]

In the USA, most states recommend a time instead of a distance, usually 2 or 3 seconds. Much easier to judge as you see the car pass some point ahead, then mentally time how long it takes for you to pass that same point. This also assumes that a car isn't going to come to a complete stop while on a highway, reducing the closing distance that would occur if the car ahead came to a complete stop, which is why the time factor isn't longer.

In heavy traffic, the larger the following distance (or time), the slower the flow of traffic (number of cars past any point per unit time), so there's also a reason to not make the following time larger than necessary for crowded freeways during peak loads, and apparently there's some trade off point due to accidents occurring because of congestion versus shorter following distances in heavy traffic. Generally drivers are told to look several cars ahead to increase reaction time.

On the interstate highways, where the traffic is less, the average driver will use much longer following distances.

On a side note, higher speed also allows for higher traffic flow. This mostly shows up during morning commutes on the freeways in some cities. The average speed is higher than normal, partially because up to 10 mph over speed limit is tolerated (not ticketed) during morning commute times in most USA cities. In the evening commute, you also have shoppers in the mix, the speed is much slower causing the traffic flow to be much less even though following distances and times tend to be less.

 

[russ_watters]

No offense, Russ, but if that's typical of US driver thinking, it's no wonder you guys keep getting into 20-car pile-ups in situations where anyone with an ounce of ability would escape safely.

You're wrong on a number of levels here Danger:

1. I don't think you realize quite how much distance you are talking about. Depending on the reaction time and static friction in the tires, it is on the order of 100m for 100km/hr. There is just no way I believe that drivers in Europe typically leave 100m between cars - and it is double the standard listed by the OP (most states in the US use a 2 second rule as others have said, which gives similar results).
2. It isn't reasonable. If it were, it would be used as a legal standard. Heck, if anything legal standards err on the side of being stricter than reasonable. If people tried to adhere to it, few roads would be drivable because of how much it would restrict speeds.
3. I don't know what news you watch, but 20 car pileups are rare and almost always involve weather (fog and/or ice). Yes, the US has a lot more car accidents than Europe - but people in the US also drive a lot more than Europeans. I'm working on the stats, but I doubt it can be said that Americans are substantially worse drivers than Europeans.

 

[russ_watters]

Just to add to the confusion on here,
Higher wind resistance occurs at higher speeds. Thus a faster car does definitely deccelerate better than an identical car traveling more slowly.

Good point, though I suspect the difference is only a few percent even between normal highway speeds and near zero. I suspect that in order to get more than a 10% difference you need to far exceed normal highway speeds.

 

[Danger]

This might be another example of culture-gap. Our country is a lot bigger than the US, with approximately 1/10th the population.
The busiest highway that I'm aware of is the 401 going through Hogtown, which, when I was living near there over 30 years ago, was 6 lanes each way. It's probably wider now. Where I currently reside, it's Deerfoot Trail. (Well, Crowchild Trail is a lot more congested, but it has fewer lanes and a lot lower speed limit.)

edit: This was composed while a couple of others were posting, so I missed some stuff. I stand by what I said, Russ, but you might have misunderstood something that I said. I was expecting that, and meant to clarify it. Under ideal conditions, I use a 3-second rule. In heavy traffic, I drop to 2 seconds. In any event, I go by what feels right at the time. Sometimes it's 2 seconds and sometimes it's 5 or 6. Where the misunderstanding might arise is that I don't drive so as to be able to stop in a straight line if the car ahead stops instantly. I do, however, make sure that in that circumstance I have time to either stop or navigate around the problem.

 

[rcgldr]

As mentioned above, the 2 second rule on a freeway assumes that the car in front isn't going brake to a full stop so the math models need to take this into account. Normally the 2 second rule applies to moderate traffic, where you can get cues from multiple cars ahead of you about any hazards that need to be avoided. If it's just you and one other car, then you should use more than 2 seconds or use a different lane, as you won't be able to see some hazardous object on the road in front of the car that you're following.

 

[mgb_phys]

The USA has a much higher level of road fatalities, either by population/car or by distance driven than the UK (figures pretty much the same for western europe)

UK 5.4/100,000 people 6.3/Billion km
USA 13.9/100,000 people 9/Billion km

This is in spite of the majority of trips in the USA being on freeways which are 5-10x safer than surface roads.

 

[russ_watters]

This might be another example of culture-gap. Our country is a lot bigger than the US, with approximately 1/10th the population.

For some reason I thought you were from the UK... I guess not. What, are you from Canada? I've driven in Canada (in Vancouver 2 months ago most recently) - it is no different from driving in the US!

edit: This was composed while a couple of others were posting, so I missed some stuff. I stand by what I said, Russ, but you might have misunderstood something that I said. I was expecting that, and meant to clarify it. Under ideal conditions, I use a 3-second rule. In heavy traffic, I drop to 2 seconds.

Ok, that is more like it - what you said implies something different:

"If it gets hit head-on by another car or truck (or even a moose if it's a small car in my region), it's stopping right now."

If a car gets hit head-on by another car of equal weight it stops essentially instantaneously. A 2 or 3 second rule is not enough to stop before hitting it. And if a truck hits a car head on, they will be going backwards toward you! It is essentially impossible to plan for these possibilities.

In any event, I go by what feels right at the time. Sometimes it's 2 seconds and sometimes it's 5 or 6.

Sure - when on a road with thin traffic, I let the cushion get very large too.

Where the misunderstanding might arise is that I don't drive so as to be able to stop in a straight line if the car ahead stops instantly. I do, however, make sure that in that circumstance I have time to either stop or navigate around the problem.

Ok...well...that's a very different situation than what we've been talking about in this thread!

 

[russ_watters]

The USA has a much higher level of road fatalities, either by population/car or by distance driven than the UK (figures pretty much the same for western europe)

UK 5.4/100,000 people 6.3/Billion km
USA 13.9/100,000 people 9/Billion km

I've seen some stats that imply the UK is unusually safe, even for Europe. The best I could do was per capita stats, though - I didn't have miles driven compared to other countries.

 

[mgb_phys]

Not much variation

Sweden 5.2 5.9
Switzerland 4.9 5.9
Norway 5.0 6.5
Holland 4.8 7.7
Germany 6.0 7.4
Finland 7.2 6.4
France 7.5 8.5
Denmark 5.4 7.7

 

[Lsos]

I'm not surprised that US driving is unsafe. I believe a lot of it has to do with the definiton of "driving". In Europe, people tend to actually drive, whereas in US, people just get in the car and let the car do the work. Lean back, crusie control, throw it into drive...and next thing you know, you're at the destination. I know, I've lived there for 20 years.

This leads to the problem of being oblivious to what's on the roads. Besides accidents, this manifests itelf in other interesting ways...no turning signals, very slow/ lazy maneuvers, the popluar "left-lane cruise" as well as the "cruise next to another car" (if there's 4 cars on a 4 lane highway, they're most likely side-by side, blocking the road).

I could spot a few reasons for this. The driver's license is one culprit...pretty much everyone gets it. The driving test centers around being able to park and turn around. Then there's the cars: a lot of them are (or at least were) very unresponsive, lacking any feedback, with a suspension that would put a hovercraft to shame, and pretty much designed with the same primary requirements as a living room couch. The result might be extreme comfort, but removes any will the driver might have had to actually take direct control of the car...

 

[Danger]

Ok...well...that's a very different situation than what we've been talking about in this thread!

On the contrary... that's exactly what the original post was complaining about. Jonny can't seem to get it through his head that speed limits and following distance guidelines are designed to minimize the chance of collision rather than just alleviate the effects of an inevitable one.
By the way, I hope that you don't think that Vancouver is in any way typical of Canada. It's a bowl of granola, like San Francisco; anyone there who isn't a fruit or a nut is a flake. It's far and away the drug capitol of the country (at least as far as importation goes), and has one of our mildest climates. Come visit me in Calgary when it's -40° C with half a metre of snow on the ground, and I'll show you what driving under adverse conditions is all about.

 

[mgb_phys]

It's a bowl of granola, like San Francisco; anyone there who isn't a fruit or a nut is a flake.

A place where, if asked by your wife to pick up Granola on the way home you don't know whether to go to the supermarket or daycare.

 

[mgb_phys]

I'm not surprised that US driving is unsafe. I believe a lot of it has to do with the definiton of "driving". In Europe, people tend to actually drive, whereas in US, people just get in the car and let the car do the work.

It's probably more to do with road usage.
In Europe freeways are much safer than surface roads, 5-10x as many accidents on A roads as motorways.
In the USA a lot more freeway driving is commuting. The freeways in LA are used very differently to a cross country autobahn.
If you could compare accident rates/km on an interstate with freeway rates in Europe I suspect they would be more similair.

 

[Phrak]

A place where, if asked by your wife to pick up Granola on the way home you don't know whether to go to the supermarket or daycare.