Re Glen's comment on following distance
When I was in Driver's training I was told one car length for every 10 MPH IIRC. This is a lot of math and I never do it., Plus I have to estimate distance. Easier to estimate time.
Practice this with a clock that has a second hand. it is best if it's the kind that "Ticks" off the seconds
One thousand 1, One thousand 2, One thousand 3, One Thousand 4
If you get the cadency right every time you say a number, you will hear a TICK
Current teaching of the American Automobile Assn for CARS on clear dry pavement is TWO seconds, ibehind the car in front of you
The RV safety folks say THREE seconds plus an additional second for every 20 feet of rig (Makes is six seconds for my outfit)
I disagree with the RV safety folks but have no problem with 3 second seperation.
This allows for reaction time, and skid distance. I know the common wisdim is "It takes longer to stop a big rig" but fact is once all wheels lock up, stopping distances are dependent on two, and only two, things 1: Pavement condition and 2: Speed (Squared no less) Pavement conditions are the same for you as the vehicle in front of you so speed squared is the deciding factor.
What does speed squared mean.. IF it takes you 22 feet to stop at 20 MPH then at 40 MPH it takes not 44 (2x) but 88 (2x2x) (Squared means multiply the speed by itself then by the distance factor) so you double the speed, the speed is quadrupled. 3 times the speed 9 times the skid)
When my wife or daughter are driving my biggest complaint is FOLLOWING TOO CLOSE BACK OFF, When I look at other drivers on the road, cars and trucks often follow at 1 second or less (1/2 second) behind the car in front, semis are running at around 1/2 to one second too. This is way too close WAY WAY too close.
How to figure your time.. When the car in front of you goes under a shadow or past a sign or other "landmark" start counting, if you reach the landmark before 2 seconds for a car or 3 for a truck, you are too close
What happens when you follow too close (And why freeway cameras are so much fun, and a good investment) This is a true story I WAS THERE IN PERSON (I am the dispatcher in this story) We got a call, "Stove in the road, S/B I-75 north of Caniff (Detroit) Well, I sent a trooper and grabbed the control box for the remotely controled cameras, called up Caniff and looked. Nothing, not even evidence of cars dodging.. So I called up Davison, same result (Davison is north of Caniff) so I called up McNichols (Six mile) and there it was, north of McNichols.. Why are cameras handy and a good investment: When the troopers I sent did not see it while going north, they would have flipped at McNichols and never seen it at all.
So I updated my troopers, called the county and we watched.. The camera was good enough I can tell you it was a GAS range, not an electric (We could see enough detail to tell the difference, burners and pigtail) I suggested we pull back a bit and watch, Now I'll tell you why cameras are FUN (At least for dispatchers and camera operators) We'd watch cars come down the 4 lane wide freeway They came in 2's and 3's for the most part. Now, if there are 4 lanes and only 2 cars. I'm not following the idiot in front of me. I'm going to move over into another lane just in case.. But for some reason these folks were right one behind the other (I know that if you "Draft" the car in front of you, you get better gas millage) Well, the lead car would change lanes at the last second, and the one(s) behind him woudl .. Well... Change underware (So far nobody hit anything other than the brakes)
Finally a group of 3 cars and a pick up came down.. Like before car one changes lanes at the last second, Car 2, underware, I'm fairly sure car three kissed bumpers with car 2, the pick up however did not slow down, swerve or stop, he place kicked that stove a good quarter mile down the road and on to the right shoulder.. I called the troopers off (no longer needed) and phoned county to move the priority down to "Tomorrow" and update them on the new location (South of McNichols, Right Shoulder)
Proper following distance would have saved a lot of underware that day
OR Courese it's not a cure all... I recall an accident on the N/B I-75 in just about the same place.. We watched a pack of cars approaching from the south, a good half mile of clear road between the pick up in the lead and the car stopped in the lane....
Did he slow down (NO) change lanes (NO) stop (NO) slam into the stalled car at a full 70 mph (YES)
Speed too fast for condition (Condition being driver not paying any attention to the road)
It got my attention - but I certainly got his attention when I laid on the horn. He and his passenger certainly looked shocked with several tons of truck and camper heading straight at them. 
