8/9/2019 Precast Beam Bridge Example AASHTO. 10/346. A2. Number of Lanes. Design Lanes [LRFD 3.6.1.1.1] Current lane configurations show two striped lanes per roadway with a traffic median barrier separating the. roadways. Using the roadway clear width between barriers, Rdwywidth , the number of design traffic lanes per. roadway, N lanes.