CMV (commercial motor vehicle) is anything with a Registered Gross Weight over 4500kgs.....distance is minimum 60metres (180ft) when travelling greater than 60km/hr
All other vehicles, you have to be able to stop in time, before hitting the rear of the vehicle in front, should the vehicle have to brake for an emergency. Use your judgement, but food for thought....
At 100km/hr a vehicle is travelling about 28 metres per second (4 car lengths of a crown vic per sec).
Once seeing the vehicle brake lights, your mind telling you to brake, foot hitting the pedal and then brakes applying (about 1-1.5 seconds) before you start slowing from first viewing.
- road conditions (pavement, gravel, uneven, curve, hill, grooves of traffic, rain, snow, ice, sand, debris)
- age of driver
- weather (wind, rain, snow, fog, sleet)
- vehicle (brake condition, tire tred depth, type of tires)
From my training, I leave a minimum 5 seconds between myself and the vehicle in front, I am also scanning for avenues of escape, our vehicles are set for regular maintenance and proper tires for the seasons, which all helps.
Another very good thing.....look ahead, not 10m, but 100-400m ahead, start seeing brake lights and prepare in advance (traffic is slowing for some reason...maybe it's a "speed trap"

)