JRscooby wrote:
I don't think that '1 car length per 10 mph' has been a recommendation for a long time. Mostly I have heard spacing given in seconds, not distance. And most do not mention that if you are following a car that can stop faster than you, you need to add time that you would be moving to the 2 seconds it takes you to react. In other words, a car 3 seconds behind a TT is likely not tailgating, but the TV pulling a TT 3 seconds behind the car probably is.
They changed the wording from "at least one car length per every 10mph" to "at least three seconds" because it is easier to figure out. There was always a question as to how long a car length is? And it's difficult for folks to determine that measurement while driving down the road. The actual distance recommendation is still the same - 3 seconds generally equates to one car length per each 10mph. But most people can do that simple measurement of counting to three while driving. It changed in the mid to late 80s.
Also, that is the mimimum recommendation. Nowadays, you'll hear " two seconds", but that was never the actual recommendation - that is a reduced distance to account for people in commuter vehicles who scream bloody murder if someone dares to merge into their lane during their commute home. It is better than no space, but closes up the open space in front of them so they are more likely to maintain it. It is used as a "bargaining chip" - "you should stay at least three seconds behind", "but all those other drivers will cut in front of me and I will never get home", "OK, then do two seconds. It's enough to give you time to react, but won't leave that huge empty space in front of you". And yes, I heard that conversation every time I took our required defensive driver course for work for the past 30 years.
In reality, you want to stay at least three seconds behind the car in front of you. Other factors should increase that distance - weather, your vehicle's capabilities, road conditions, any impairment to your response speeds, etc.