Traffic lane is the number of lanes of traffic that a traffic engineer plans for bridge or road. It is part of carriageway designated by proper marking and is used to control and guide the driver (typically it is 3.6 m (12 ft).
Design lane is the lane designation used by the bridge engineer for live load placement, which 3 m (10 ft). It consists of uniformly distributed load of 0.64 Kips/ft and is assumed to occupy 10 ft transversely. The design lane width may or may not be same as that of traffic lane. AASHTO LRFD uses a 3 m (10 ft) lane for design in which the vehicle is positioned for extreme effects. The number of design lane can be taking the integer part of the ratio of the clear roadway width by 3.6 m (AASHTO).