Feel free to offer explanations!
In what under what conitions, weight, weather etc…?
It depends on the situation but if you leave a safe distance its normally filled by suicide jockeys
Hombre:
In what under what conitions, weight, weather etc…?
M6 56MPH 40Tonne
From experience 10ft seems about the average 
double to 20ft if poor weather conditions should do it
remember, only a fool breaks the two second rule.
or if you prefer
only a [zb] hits the car in front
you should be able to smell the paint/plastic on the vehicles bumper in front of you at all times 
SPEED STOPPING DIST SAFE FOLLOWING DIST SECONDS
10 mph 14 ft 44 ft 3.0 sec
20 mph 44 ft 100 ft 3.5 sec
30 mph 88 ft 175 ft 4.0 sec
40 mph 146 ft 263 ft 4.5 sec
50 mph 220 ft 365 ft 5.0 sec
60 mph 306 ft 482 ft 5.5 sec
70 mph 409 ft 614 ft 6.0 sec
copied this from another site
speed… stopping distance…safe following distance… time to stop
claimed to be for LGV’s but doesn’t say what size or if loaded.
truckerjon:
only a [zb] hits the car in front
Quality.
Ken.
As a general rule if I’m on the limiter and catching another trucker in front doing slightly less than me, I’ll leave it til about an artic length away before pulling out. If I’m following a truck who’s doing the exact same speed as me then I want about 3 artic lengths space, minimum, to be comfortable.
Depends how well I can see the road in front. If the road is fairly curvy then I’ll go closer 'cause you can see what going on up ahead, but if it’s a straight road then I want some space please.
the rule in europe is 50m but at least one artic length before overtaking.
you can’t ask how many car lenghts as a mini and a mercedes e class estate are a big difference in length.
did you get the idea of this from the WUM mr clark on another post? he claims that he can’t see enough to do more than 55mph but thats 220ft stopping isn’t it? needs glasses i think
It’s amazing just how many people can’t see past the end of their bonnet simply from having bad eyesight. I’m not one to preach here as the last eye test I had about a decade ago dictated that I needed “correction” on my right eye I think it was. That said, I have no problem seeing traffic clearly 1 or 2 miles up the road nor any problems reading the junction signs from c.800 yards away, but I sometimes struggle to focus on the text on my screen in front of me here 
. Anyway… I was travelling as front passenger with my older bro a few weeks back coming up Scott Hall Road out of Sheepscar. Traffic was a bit busy and he was in the outside lane of the DC doing about 40. Someone up ahead about 10 cars in front had decided to turn right into the estate and all the cars in front braked to halt fairly sharply as the lane became blocked. My bro just kept on going though, even though though the traffic had stopped. I had to shout out as we were rapidly approaching the back of the stationary car about 40ft in front. I was like “WTF is wrong with you?” and he admitted that he hadn’t even seen the traffic had come to a stop . It later transpired when we were on the motorway that he couldn’t even read the writing on the motorway junction signs from 3-400 yards away - “it’s just blurry writing from here” he says. He didn’t think there was owt wrong with his eyes until we had a little ‘competition’ of who could read the signs first, and it was only then that he realised his eyesight was truly bolloxed and went and got some specs. I reckon there’s a WHOLE BUNCH of other folks like that on the roads, which is quite scary tbh.
It depends on the coeficent of friction on the road plus weather conditions,speed weight, and if you are ogling some bird and not looking where your going.The molecular structure of the tyres may be a factor if it is a hot day or a cold day and if the correct pressure is in them,it may be the way the truck is loaded wrong axle weight ect etc,me I have a good idea what my truck needs to stop in ,it is a bit like when you kick the tyres you have a good idea if they are ok.
Q.What speed ?
If your asking the question of when stopping in queuing traffic or approaching a junction or traffic lights or coming to a controlled stop 
, and leaving a safe sufficient gap in order to make a manoevere slow moving traffic flow Vehicle in front slows/stops to make a turn
A. at least your vehicles length in order to be able to manoevere around them in case they stall or breakdown.
otherwise , answer is :
Lots of variables,
The typical stopping distance as a general guide at 70mph is 96 mtrs (315 feet) ( variable depending on your attention-thinking distance, the road surface,the weather conditions and the condition of your vehicle at the time.
and some can vary on your speed - attention- road surface + -weather conditions + visibilty - traffic conditions- what size /type /weight vehicle your driving + how many pies you ate .
General rule of thumb :- 2. > . > . Chevrons apart 
(100 mtrs) , which equates to approx` 2 seconds gap @ 54 mph in a truck or 70mph car/van
(if you cant make 54mph - tuff - just leave a bigger gap ).
As a guide On motorways/dual-carriageways = 1 verge roadside marker apart (markers are 100 mtrs apart).
Unless its icy or your aqua- planing (lots more distance than 96 mtrs ), in which case, tuck your head well down and kiss your ■■■ goodbye
we have for car’s by good weather 2 second,Car by Bad 4 second minimum
Lorries minimum 4 second
to Vehicle in Front 
Now. how much is 4 second distance from Vehicle in Front?
That’s bit Mathematic 
You have if you count in Kilometer for each Kilometer 1000 Meter,and for each Hour 60 Minutes or 3600 Second
We say we drive 50 Km/h (30mph) thats 50000 : 3600 = 13.9 Meter/Second
Thats 56 Meter by 4 second
but no one needs that by such Speed as the Queue would be too long.
Now,by 80 Km/h were that 80000 : 3600 x 4 = 89 Meter
by 96Km/h (60 mph) 96000 : 3600 x 4 = 106.6 Meter
It’s logically. So faster you drive so bigger is the Gap by 4 Second,but you have to Quartile it by Ice.
Safer to just leave a bigger gap really ,
then watch johnny no-brain slip into it as they dive from another lane and go under your bumper as your braking 
Two vehicle lengths for each 50km/h of speed.
Driving car in a town (30mph) - leave gap for two cars; on motorway at 60mph leave gap for 4 cars. Driving lorry - leave a gap for 2 lorries in town, for 4 on motorway.
Bad weather - slow down.