This page clarifies the parameters on the "Define Bridge Section Data" form.
Live Load Curb Locations
- Distance To Inside Edge of Left (or Right) Live Load Curb: The form measures from edge of deck, whereas the code measures from centerline of the exterior girder. It is for this reason that the value cannot be negative. The AASHTO value is d_e = x_e - x, where x_e is the distance from the edge of deck to the CL of the exterior web, and x is the user-specified distance to the edge of deck.
Figure 1: Live load curb locations