Roman engineering was mostly synonymous with military engineering. Those roads that they're so famous for weren't built so much for day-to-day use (though they were, of course, useful for that) as for marching legions quickly into the countryside, hitting trouble spots and getting out again. Roman-designed pontoon bridges, constructed mostly during wartime for the shock and awe of quick raids, served the same purpose and were a specialty of Julius Caesar's. In 55 B.C., he built a pontoon bridge that was around 437 yards (400 meters) long to cross the Rhine river, which was traditionally thought by the Germanic tribes to be safely out of reach of Roman power.
Caesar's Rhine bridge was clever for a couple of reasons. Building a bridge without diverting a river is notoriously difficult to do, and even more so in a military setting where construction must be guarded at all times, so engineers had to work fast. Rather than driving beams straight into the river, engineers rammed timbers into the bottom of the river at an angle against the current, lending the foundation extra strength. Protective pilings were also driven in upstream to catch or slow down any potentially destructive logs that might float down the river. Finally, the beams were lashed together, and a wooden bridge was built on top of it. In total, the construction took only ten days, used entirely local lumber and sent a firm message to local tribes about the power of Rome: if Caesar wanted to cross the Rhine, he could do it.
There's also the possibly apocryphal story of Caligula's (yes, that Caligula) pontoon bridge built across the sea between Baiae and Puzzuoli, a roughly 2.5-mile (4-kilometer) span. Supposedly, Caligula commissioned the bridge because a soothsayer had prophesied that he had roughly the same chance of becoming emperor as he did of crossing the bay of Baiae on a horse. Never one to practice restraint, Caligula allegedly took it as a dare, lashed a chain of boats together, covered them with dirt and went for a ride.