The traffic may depend on the construction situation. Before you leave, check the traffic page at washingtonpost.com to see if there is any scheduled construction on the route you plan to take.
Another thing to keep in mind: even a good traffic day on the Beltway can be nervewracking to someone who isn't used to it. You may not have to go slow, but there will still be plenty of cars packed onto the road, and the locals are *aggressive* about lane changes. It's worse if you are a courteous driver, because everyone on the road will see that you are the only chance they have to change lanes, and will cut you off repeatedly.
Not to scare you, but if you have kids you might want to prepare them to be still for that short period of time so you can concentrate on the road.