Beginning this month, King County Metro Transit will add paid advertising to its Downtown Seattle Transit Tunnel .

The advertising is designed to help offset revenue losses resulting from the faltering economy.

The tunnel ads have the potential to generate more than $230,000 annually. When combined with sales of ads on bus interiors and exteriors, the ad revenue could climb to more than $6.6 million annually – the cost of keeping 60,000 hours of bus service on the street.

Titan Outdoor contracts with Metro to sell advertising space on the agency’s 1,300 buses, and will handle the tunnel advertising.