The most spectacular feature of the Francolí aqueduct is the aqueduct bridge of Ferreras, also known as the “Pont del diable” (“Devil’s bridge”) with a maximum height of 26 m and a length of 249 m, including the ends where the specus runs on a wall. It consists of two tiers of arches, all with the same diameter of 20 Roman feet (5,9m) with a variation of 15 cm. The distance between centres of the pillars is 26 Roman feet (7,95m). The present width of the specus is 2,5 roman feet, but this may have been modified by restorations in the course of time. A special feature of this aqueduct bridge is the stepped nature of the piers below the imposts of the arches. Each step increase the width of the pier by half a roman foot (15 cm), almost certainly to increase stability. The bridge probably dates from the time of Augustus. There is a small quarry 100 m North of the bridge which may have provided the building stone.
The Ferreras aqueduct bridge can be easily reached from the N 240, but also from a parking on the A7 motorway (only for travellers coming from the direction of Barcelona!). From the parking, it is possible to walk in the specus over the aqueduct, and also to reach the valley below.

You may read complete info here:
http://www.romanaqueducts.info/aquasite/tarragona/