King County plans to purchase up to 500 new buses that will help deliver one of the biggest bus service expansions in Metro history.

Most of the buses will be articulated hybrids manufactured under a contract believed to be the largest of its kind in North America.

Metro has selected New Flyer Industries, General Motors Corp. and Cummins Engine Co. as the major commercial partners in delivering the next generation of buses. All the buses will meet or exceed the latest federal environmental standards.

The first 22 articulated hybrids will arrive next spring, with another planned order for 100 buses in 2009 to provide new Rapid Ride service on five routes.

The contract will allow Metro to place multiple bus orders over the next five years to support its Transit Now initiative that expands service up to 20 percent. The expanded and more frequent service is designed to make transit more attractive to drivers. Metro's goal is to get up to 50,000 drivers out of their cars and riding buses by 2016.

The contract, structured similarly to those used in the aviation industry, will give Metro the flexibility to order different types of buses and components specifically designed for different uses, whether it is hybrid-electric, regular diesel-powered or European-style coaches fashioned for future bus rapid transit routes. General Motors and Cummins will provide major operating components for the buses.

Metro expects subsequent orders will be used to replace aging buses in its fleet and for expanded service to offset the traffic impacts associated with reconstruction of the Alaskan Way Viaduct and State Route 520.

While the cost of the new buses will depend on the types of features and components ultimately selected, the order could be worth as much as $400 million over the life of the contract. The buses will be paid for with a combination of local, state and federal funding sources.