I'm sure BDC is aware of this, so I'm surprised he didn't mention it, but the Twin Cities metro area has a very extensive system of bus-allowed shoulders, where buses can use the shoulder during times of congested traffic. This is supported by state law, which governs the use and speed limits of bus shoulder use (namely a 35 MPH speed limit, but still faster than traffic going 10-15). MnDOT has been very supportive, and has undergone a program of shoulder improvements and minor shoulder widening to better handle the buses using the pavement. This improvement program isn't just for the freeways...several at-grade arterials have had their shoulders improved for bus use. These improvements helped out what was already a successful express bus system in the Twin Cities metro...the I-35W South (i.e. south of downtown Minneapolis) express buses alone carry 15,000 passengers a day.Metro Transit.
MnDOT has also recently started implementing what they call a "Dynamic Shoulder Lane" on I-35W. They got the idea for the concept from VDOT and what they do on I-66. In their test case (which got UPA money a few years back from the Feds), the inside shoulder on northbound I-35W near downtown was reconstructed and upgraded...during normal times, it's basically a shoulder. During peak hours, it's basically a HO/T lane. Buses and HOVs can use for free, and solo drivers can pay to use if they have a transponder.
Unfortunately, most of the congested freeway segments in the DC area lack an inside shoulder, so similar application in DC may be limited. However, if VDOT can find a way to fully reconstruct I-66 between Fair Oaks and the Beltway (as they should've done 20 years ago), they could limit the major reconstruction to that segment and convert the wide inside shoulder west of Fair Oaks into a dynamic HOV or HO/T lane. This would allow for not just 1 but 2 HOV (or HO/T) lanes on I-66 in the peak direction between the Beltway and Gainesville (assuming the Fair Oaks-Beltway reconstruction allowed for 2 HOV lanes). The potential issue here would be enforcement, since I-66's HOV lanes are already heavily violated. But at the same time, MnDOT found that HOV violations on their I-394 and I-35W lanes dropped in half when they converted the lanes to HO/T lanes.
It's a concept that the DC area would do well to consider and implement.