The Central Artery-Tunnel (Interstate 93) through downtown Boston, Massachusetts, is being reconstructed as a tunnel 2.4 km (1.5 mi) long. Within the tunnel several exits require guide signing for motorist safety and efficient traffic operation. Design and placement of this signing bring challenges due to the tunnel's low ceiling height of 5.19 m (17 ft) and horizontal and vertical curvature that reduces the distance from which motorists can see the signs. Tunnel guide signs may also be blocked from view by large trucks, due to the lower than usual mounting height. Reconstruction of the Central Artery will improve traffic operations by reducing the number of exits and entrances on Interstate 93 from the current 27 exits and entrances (two-directional total) over 3 mi to 14. Yet, the tunnel will have 7 exits, 3 of them compressed into a southbound tunnel segment 1250 m (4,100 ft) long. These conditions may cause drivers--especially unfamiliar drivers--to have difficulty obtaining guidance information for their exit. This can lead to driver frustration and a reduction in safety caused by abrupt lane changing and other maneuvers. To address these problems with improved sign design and placement, a study using a driving simulator was undertaken. A computer-generated roadway through the tunnel was developed to replicate the tunnel geometry (including horizontal and vertical curvature and ceiling height) and sign placement. Test subjects drove through the simulated tunnel to evaluate the developed signing alternatives. The driving simulator was also assessed as a tool for improving freeway guide signing.