Pontifex does indeed mean 'bridge builder', but the Pope did not gain the title because of any responsibility for bridge repairs. The title had previously been that of one of the highest-ranked priests in Rome. The Pontifex Maximus - 'greatest mediator' - was responsible for important aspects of the civic cult. (I'm not sure what aspects specifically - this is all from memory.) Julius Caesar was granted the title during his time as consul, and when his great-nephew and adopted son Octavian became the Emperor Augustus, the role was absorbed into that of Emperor, leaving the Flamen Dialis as the highest-ranked priest in the Roman cult.