Callahan Tunnel
The Callahan Tunnel, officially the Lieutenant William F. Callahan Tunnel is one of four tunnels beneath Boston Harbor in Boston, Massachusetts. It carries motor vehicles from the North End to Logan International Airport and Route 1A in East Boston. Ordinarily, this tunnel is only used to carry traffic out of the city, and with the completion of the Big Dig it only collects traffic from I-93 southbound (right after traffic merges from Storrow Drive) and downtown Boston. There is no toll in this direction. Traffic from the airport and Route 1A towards downtown Boston and I-93 northbound normally flows in the older, parallel Sumner Tunnel, where a $3.50 toll is collected (raised from $3 on January 1, 2008).

Traffic flowing between Logan International Airport and directions south of the city on I-93 and west of the city on the Mass Pike (I-90) normally uses the Ted Williams Tunnel rather than the Callahan and Sumner Tunnels.

The tunnel was opened in 1961. It was named for the son of Turnpike chairman William F. Callahan, who was killed in Italy just days before the end of World War II. Operatic Tenor, William Flavin, of Milton Mass. sang the Star Spangled Banner and Oh Danny Boy at the opening of the Callahan Tunnel in 1961.
Historically, control signals were used to reverse direction of one lane in this tunnel or the Sumner Tunnel, when the opposite tunnel was closed for maintenance or emergencies. Under the relevant Turnpike regulations, a yellow signal light means "proceed only as directed", on penalty of a $50 fine. As the signals are almost always yellow, this rule is universally ignored by drivers. Other markings in the tunnel include a "double white line" in the center, intended to discourage drivers from changing lanes, to be penalized with a $100 fine.