This paper examines the design of high performance transit network that minimizes the transit agency costs, the time of transit users in the system and the travel time of car users. Surface transit services need segregated lanes to achieve a target cruising speed, so that the transit network length has a significant impact on the traffic performance. This effect has been modeled in this paper by the Macroscopic Fundamental Diagram (MFD). The optimization process encompasses two different approaches. First, an analytic model defines the optimal layout of surface transit networks according to the time headway, stop spacing, line spacing and network size variables. Then, the average travel time of the traffic network is evaluated by the use of the MFD curve corresponding to the former transit network length. The full methodology has been implemented in the city of Barcelona. The calibration of MFD in its central district has been done by means of traffic microsimulation runs. The social optimum number of corridors is 15, somewhat smaller than the optimization which does not consider traffic (23).