The long-awaited San Francisco-Oakland Bay Bridge officially opened on November 12, 1936; construction had begun in July 1933. Though he did not attend the opening ceremonies in California, President Franklin Delano Roosevelt (FDR) was an active participant. While in his White House office in Washington, DC, FDR pressed a button that switched on a pair of flashing green lights on the bridge and thus officially opened it to traffic. FDR pushed the button on his desk at Noon; the green lights were located at the midpoint between San Francisco and Oakland - the two cities linked by the bridge.
Locally, California Governor Frank Merriam participated by using an acetylene torch to melt a golden link that held together a chain that spanned part of the bridge's roadway. The Governor's actions completed the official/physical/ceremonial opening of the bridge (though celebrations would continue in the cities for a couple of days).
Officials estimated that 80,000 vehicles crossed the bridge during the first 12 hours after its opening; it was further estimated that 350,000 passengers were in the cars that made the trip. Approximately $50,000 was collected in bridge tolls from the "early adopter" vehicles.
Though authorized as a commemorative coin in recognition of the bridge's opening, the coins were not physically available for the opening. The coins were not struck by the Mint at San Francisco until early November and were not delivered until after the official opening. The coins were, however, made available before the end of November at booths near the bridge entrances (and possibly at toll booths for the bridge, contemporary reports differ) The cost at the booths was $1.50 per coin (the price paid for the commemorative coin
did not include the bridge toll).
1936 San Francisco-Oakland Bay Bridge Half Dollar

For other of my posts about commemorative coins and medals, including more history on the Bay Bridge half dollar, see:
Commems Collection.