Talgetting is measured in seconds as reaction time + braking distance = ~2s, whether you are on a highway or in a city. There is nothing sujective about it. Most drivers keep just enough room for the reaction time, and it works ok as long a the car ahead doesn't hit an immobile object (e.g. a log, a car, a kangaroo, a wild boar).
Fixed 1 second of braking time won't save you if the front car hits an immobile object.
The correct formula would be (1 + .046 mph) seconds [0], where mph is speed in miles per hour. It evaluates to 3.76s at 60mph and at 4.9s 85.
Note that 1 second of reaction time is not exactly generous: http://www.webpages.uidaho.edu/niatt_labmanual/chapters/geom... - cites times 'less than 2.5 seconds' to actual brake application.
[0] 1mph/(9.8m/s2) in seconds, according to Google, assuming friction coefficient of 1
