As you enter National Airport coming from the GW parkway south, airport police sit on access road off to the right hand side; right before you turn off for rental car return and entrance to terminals. Speed goes from 45 to 25 as you cross under GW parkway. They are trying to nail people rushing to flights or distracted tourist. Tourists are easy prey since they are unlikely to return to DC just to argue a ticket. I live here and got caught going 42 in a 25. Been back several times around the same time (6-8pm) and sure enough he is there waiting.