Beats me. I think it would make more sense to make it reversible like the I-95 HOV lanes from Garrisonville to Springfield in northern Virginia.
No, no, no -
A THOUSAND TIMES no!
The I-95 express lanes in Virginia would do much more good if there were separate non-reversible lanes.
They are reversible only as a legacy of the lanes that are mostly on what is now signed as I-395 (formerly I-95 from the Capital Beltway into D.C.), which had and has a strongly directional flow of traffic north in the AM and south in the PM. In the early days, those lanes came to an end just south of VA-644 in the Springfield area of Fairfax County.
In the 1990's, the reversible lanes were extended as far south as VA-234 near Dumfries in Prince William County (HOV-3 restricted during peak demand hours), then Transurban, the private concession holder, extended the lanes south to Garrisonville, currently south of VA-630). The extended reversible lanes have worked well, but they are not much help with weekend or holiday traffic on I-95, when
both sides of I-95 could use the added capacity.
The further away from Arlington County, Virginia we get, the more the off-peak side could use the added capacity. This is in part because of the BRAC process (Base Realignment and Closure Commission), which has
significantly increased employment (and traffic) at both the U.S. Army's Fort Belvoir and the nearby Fort Belvoir North Area in Fairfax County. Same story at Marine Corps Base Quantico in Prince William and Stafford Counties, which has experienced a lot of added employment because of BRAC.
Getting back to Maryland, the Defense Department facilities in Harford County (the former Edgewood Arsenal and Aberdeen Proving Ground) have also had a net gain of jobs because of BRAC.