Access from West Germany to Berlin was done through a single road and train route that ran directly to the city. Both the road and trains were under scrutiny with the road having regular stops and checks that were timed. If it took too long to travel from one checkpoint to another you could be detained, this was the same route that was used by military personnel to travel to Berlin. Military personnel were instructed to not leave their vehicles and to only deal with Soviet soldiers and not DDR soldiers.

Stars and stripes have an article on it which can be found here