FYI - from the DC Metro website:
www.wmata.com
Daily parking at Metro stations
Monthly parking information
General
Note: SmarTrip® cards with full parking fees are the only accepted form of payment for parking with the exception of: Anacostia, Franconia-Springfield, Largo Town Center, New Carrollton, Shady Grove and Vienna/Fairfax-GMU, where major credit cards are also accepted. You will not be able exit any other Metro parking lot unless your SmarTrip card has enough money on it to pay the full parking fee.
See the SmarTrip page and SmarTrip parking brochure for details on buying and using SmarTrip. Please note that SmarTrip cards are also sold through vending machines at all stations where parking is available. See list.
Note: Credit Card pilot program
Paying to park has gotten easier. Customers now can pay the parking daily fee with a major credit card at six pilot Metrorail stations. The parking facilities at the Anacostia, Franconia-Springfield, Largo Town Center, Vienna/Fairfax-GMU, Shady Grove and New Carrollton Metrorail stations each have one exit lane that accepts credit card or SmarTrip card payments. Customers are able to use their Discover, Mastercard, Visa, American Express or Japanese Credit Bank cards at these locations. A credit card reader is installed next to the existing SmarTrip card target reader on the side of the booth within the appropriate lane.
Parking at Metro-operated lots is FREE on weekends and most federal holidays. For details, see Metro's holiday schedule. On holidays for which Metrobus service is indicated as being on "weekday schedule," normal parking fees apply.
All-day parking, up to 24 hours, is available at all stations that have Metro-operated parking lots (see list).
Note: There is no charge for multi-day parking beyond the regular fee (paid with a SmarTrip card) on the day of exit, if customers exit during collection hours (10:30 a.m. - midnight on Monday - Thursday, and 10:30 a.m. - 3:00 a.m. Friday). Multiple day parking is available at three stations: Greenbelt, Huntington and Franconia-Springfield. Each of these stations has between 15 and 17 spaces allocated for multi-day use of up to 10 days, which are available on a first-come, first-served basis.
These spaces, located inside the regular facilities and not at the metered (kiss and ride) spaces are at:
1. Level 1J at Franconia station
2. The South Lot at Huntington station (This is the lot, not the garage off N. Kings Hwy)
3. Cherrywood Lane side at Greenbelt station
Fares are collected upon exit at most stations, between 10:30 a.m and closing time of the rail system. (See all collection hours by station). Suburban lots and garages often become full by 8 a.m. on weekdays. Space is generally available all day at the following stations: Landover, PG Plaza, Twinbrook, White Flint, and Wheaton.
