This is a classic speed trap: A multi-lane 'sub-grade' highway with no pedestrians and highly variable traffic (often backed-up during 'rush-hour', but mainly empty at most other times, especially at night). So, unsuspecting drivers will tend to treat this road as a normal highway, but it has an unexpectedly low speed limit of 40 mph and a new speed camera. There appears to be no elevated risk to pedestrians or to other cars, so the camera is simply a money-raising exercise for DC. And speed cameras are very important to the DC budget. Five times more important than parking meters (in a heavily metered city). Unfortunately, since we live just outside DC, we have no political influence!

Just got a $100 ticket for driving at 50 mph. Never saw any sign.

I just Googled it and yes, there's a sign for [Speed Limit 40 - Photo Enforced]