Sometimes i can't tell if a car has stopped in front of my car.?

ill be driving on the road, and sometimes the car in front of me will have its red brake lights on..after i stare at the lights, i seem to not perception that the car has stopped. i think that if i stare at the red lights too long, its like watching a train straight ahead..you cant tell if the train is coming towards you when you see it directly in front of it..likewise, when the red brake lights are on, i cant tell if the car has stopped, but until i turn away and look at something else then go back to watching the car and stopping my car safely.