US19 is a 4 lane divided highway between the Cranberry Twp border on the north (where PA turnpike and I79 converge with US19)for several miles south to the Wexford business district. The speed limit is set at 40mph, while adjacent 2 lane curvy roads have a 35mph speed limit. This stretch of highway is well-known locally for being a speed trap but is recently being even more strictly enforced (see http://www.pittsburghlive.com/x/search/s_189937.html).
Some monitoring is necessary as there are some dangerous spots (red lights at bottoms of hills, where traffic from neighborhoods comes out and could easily be hit be a speeder running the light), but on lengthy stretches with no access, a 55mph speed would be totally reasonable.