They say you should allow enough time for a journey that includes reasonable delays. When does 'reasonable' become a joke. A simple 35 mile trip was plagued by ditherers, middle and outside lane hoggers and worst of all what I call the ten-unders, those people who no matter the speed limit insist on driving at 10mph below it. Now I know this will incur the ire of the pedant but when I took my motorcycle test relatively recently the instructor and examiner both said how important it was to drive at or as close to the 'speed limit' as was safely possible, proving that it wasn't a limit but a target that helped remove chaos.