[kbutler]

> Suggested highway following distance is two seconds.

Suggested by tailgaters?

1) A two-second rule is insufficient.

2) You need more time if you can't see around or through the vehicle ahead of you (and various other reasons, like weather, road conditions, being followed by a tailgater, etc.)

California driver handbook: """ Most rear end collisions are caused by tailgating. To avoid tailgating, use the “3 second rule”: ...

You should allow for 4 or more seconds or when:...

Following large vehicles that block your view ahead. The extra space allows you to see around the vehicle. """

[jws]

Most US States say two seconds. Some say three. German law requires 0.9 seconds, but studies show 41% of drivers there follow closer in relaxed conditions!†

Out here in the relaxed midwest most drivers in 60mph traffic seem to follow about 1.5 seconds.

It looks like highway planners expect to get 1900 vehicles per lane per hour at saturation, that averages 1.9 seconds between cars.

␄

† https://en.wikipedia.org/wiki/Two-second_rule (wherein it proclaims sleep impaired drivers in bad conditions should leave 6 seconds). If you think you need a six second reaction time, pull over and get out of the car.

[kbutler]

I'm curious how you've determined "most US States". I cited CA above - 3 or more, Washington wants 4 seconds above 35 mph. Florida says 4 seconds during normal weather and traffic conditions (more if hazardous). Utah does say 2 seconds. Texas wants 4 seconds above 30mph. New York said 2 seconds but "In bad weather and when following large trucks, increase the count to at least three or four seconds for additional space."

And in any case, the 2 seconds is not the "suggested follow distance" it's the legal MINIMUM. That is, "You're clearly unsafe if following at any less than this," rather than, "Close up any gaps more than this."

[qznc]

The rule of thumb in Germany is "half speedometer" which corresponds to 1.8seconds. For example, if you drive at 80km/h, keep a 40m distance. The reflector posts [0] on streets out of towns have a 50m distance, so the distance between two posts at 100km/h.

[0] https://de.wikipedia.org/wiki/Leitpfosten

[barrkel]

At 80mph, 2 seconds is about 240 feet (72 metres); 3 seconds 360 feet (108 metres). I'd expect to have my safety buffer merged into with a three second gap. It can still happen with a 2 second gap, but it's a lot less likely.

[kbutler]

So you are willing to reduce your safety to stay ahead of the guy in the other lane.

Traffic flows best when drivers allow merges, not when they try to close gaps to prevent merges. Most aggressive mergers will quickly merge out as well.

You may even get some "thank you" waves...

[barrkel]

I'm not talking about closing gaps to prevent merges, but leaving a compromise between continuously merging traffic in front of you - requiring you to slow down continuously but randomly - vs maintaining a steady speed with a predictable buffer.

[sangnoir]

> I'd expect to have my safety buffer merged into with a three second gap.

Does this happen frequently enough for you to ignore safety concerns? If so, you should be able to live(or not) with the tradeoff you are making.

[barrkel]

The large gaps mean you need to continuously slow down for the people merging in - overall it decreases safety, IMO.