Beginning next week, travelers can no longer leave their cars at four Metro stations in Fairfax County when they plan to be out of town for more than 24 hours, transit agency officials said yesterday.

Metro, acting at the request of the Fairfax County Board of Supervisors, will prohibit parking for more than 24 hours at the Huntington, Vienna, Dunn Loring and West Falls Church stations. Vehicles left more than 24 hours will receive $25 tickets and be subject to towing, said Marilyn Dicus, a Metro spokeswoman.

The transit agency has started erecting signs warning of the new policy, but will not start enforcing it until all the signs are up, sometime next week.

Metro board Chairman Joseph Alexander, a Fairfax supervisor, requested the policy after receiving complaints from commuters that the parking lots are full before the end of the morning rush hour and some cars are left in the lots for several days.

County officials have suspected that air travelers are using the lots to avoid parking fees at National Airport, pointing to the difference in cost as an obvious incentive: Parking at National costs up to $20 for 24 hours in the short-term lots, up to $7.50 for 24 hours in the daily lots, and $6 a day in the satellite lots; Metro parking in Fairfax is $1.50 a day for cars leaving the lots between 3 and 10 p.m., and is free for cars leaving before 3 p.m.

Thus, before the new rules, a traveler could leave a car at one of the Metro lots for an indefinite period of time and pay nothing by retrieving the car before 3 p.m.

Alexander said that, given the shortage of Metro parking, it is "just unreasonable" to tie up spaces that daily commuters could use.

In a separate development, Metro officials said they expect federal safety specialists to recommend improvements in Metro's alarm system to ensure that Metro trains are automatically stopped when a freight train derails onto Metro tracks.

Metro's warning system is triggered when a person or object knocks down the fence between subway tracks and railroad tracks. The system alerts Metro officials downtown to stop trains headed toward the site of the incident.

A direct electronic link between the warning system and the trains' control system would stop the trains automatically, bypassing the officials downtown and reducing the possibility of human error.

The cost of the linkage, as well as other improvements to the warning system, may cost up to $2 million, and could be installed 18 months from now, said John Flynn, Metro's deputy assistant general manager for rail operations.