1. The posted speed limit approaching and leaving the main toll plaza at the Beltway is 35 mph.
Even though adhering to the letter of the law may be nigh-suicidal (especially in the E-Z Pass only lanes), failure to comply, or at least failure to stay under 55mph (20 over the posted speed limit) could lead to a reckless driving citation.
2. If you're heading west from I-66 and, perchance, happen to get on the Dulles Airport Access Road instead of the Dulles Toll Road, you should just take your lumps, head out to the airport and turn around, obeying all traffic laws and maintaining a moderate speed.
This is especially true if you're driving at 2am and there's no one else on the road.
If do you find that you missed the turnoff and are on the airport access road, under no circumstances should you try driving in reverse all the way back to the split.
This goes double if you're 3 football fields past the turnoff, like that moron I saw last night while I was coming back from Arlington.
3. Heading inbound during the morning rush, and thinking of going to Dulles, just so you can turn around and take the access road eastbound?
Ever notice a line of cars pulled over on the eastbound access road, surrounded by traffic cones and police cars?
They keep track of the license plates of the cars which do this.
Now, I didn't say that these were necessarily going to be my hard learned lessons: I've only done the firsthand learning on one of these three things. I got the scoop on the other two from folks I know.