The 12.9-km-long Shea-San Tunnel, located on Taiwan's 54-km-long National Highway 5 (NH-5), has two one-way tubes. To minimize accidents and to respond to the need for improved operating efficiency on NH-5, Taiwan's Bureau of National Highways adopts a graduated approach in traffic control. Before October 2007, the bureau set a uniform speed limit of 70 kilometers per hour (km/h) for NH-5 and allowed only passenger cars and other small vehicles to enter the Shea-San Tunnel and the adjacent Pern-San Tunnel. With the exception of the Shea-San Tunnel, the speed limit was raised to 80 km/h in October 2007. In November 2007, the bureau opened all the tunnels to commercial buses and, most recently, the bureau raised the speed limit in the Shea-San Tunnel to 80 km/h. One traffic regulation that remains unchanged is that motorists in the Shea-San Tunnel must maintain a minimum car-following distance of 50 m under normal conditions. The capacities of the two tubes in the Shea-San Tunnel are still low. This study uses detector data to examine the spatial and temporal variations of capacity, free-flow speed, passenger car equivalents of buses, and speed–flow relationship.