In my eight months in DC, I've commuted to work by bus, metro, car, private shuttle, and by carpooling with strangers (a.k.a. slugging). The only major transit option I've not used is the train, mainly because it's a good 20 minutes slower than slugging. With my vast experience in commuting, I can make one definitive conclusion: they're all full. The roads are full, the buses are full, the slug lots are full, the metro is full, the shuttles are full, and I've heard the trains are full too.
The long-term solution would be to lift DC's height restrictions
. In the short term, since alternative work schedules are already popular, there's not much that can be done to reduce the rush hour flow. Therefore, building more capacity is paramount. The trouble is that, while you're building this capacity, there's no where for the diverted traffic to go. For example, you can't move everyone onto buses while you widen a metro tunnel because the roads are full! You have to be able to build capacity without disrupting traffic flow so that you can undertake projects that do disrupt traffic flow.
So here's my plan:
First, build another few slugs lots. Since many lots are full half-way through rush hour, this would encourage more people to slug, especially since these lots will be more convenient for some. It would also encourage more alternative work schedules. However, the HOV lanes are often full once near the city, which leads to the next step:
Second, lengthen the restricted times on the HOV, and provide a variable minimum capacity depending on the time. During the peak rush hour commuting time, instead of the standard 3+ occupancy, you change it to 4+. This should put a lot of cars into the extra slug lots. This would make the HOV a much more viable option, but the roads would still be clogged, which leads to the next step:
Third, establish a rush-hour toll on the non-HOV lanes. Besides encouraging more people to slug and take buses (whose capacity could be upgraded rapidly), this would then fund the upgrades to other things, such as:
Fourth, add express trains/tracks in the metro. The outer metro stations are under-utilized because it takes so long to get there with all the stops in between. Instead, send an express train straight to the farthest station on the line (or the city center). People a couple of stops short of that could then board a regular metro train to get to their station. This would also use the under-utilized reverse direction trains, e.g., the p.m. inbound.
Fifth, once they have money and spare capacity, I'm sure they can think of other things that need upgrading.
So in short, you maximize the use of the cars that are already going in, charge the people who drive by themselves during rush hour, and use the proceeds to fund the expansion of other mass-transit options. That's my plan, anyway. Incidentally, expanding slugging opportunities plays right into my preferred mode of commuting.