Leave enough space between you and the vehicle in front so that you can pull up safely if it slows down or stops suddenly, the safe rule is never to get closer than the overall stopping distance shown below, but on the open road in good conditions leave a gap of one metre for each mile an hour of your speed or a two second time gap may be enough, they will also leave space for an overtaking vehicle pulling or wet or icy or the gap should be at least double that, drop back if you know that taking vehicle closing gap in front of you  make way for ambulance, fire engines, police or van emergency vehicles when their blue lamps are flashing or their bells, two tone cones or sirens or .