The city of Philadelphia is known for its very dangerous Schuylkill Expressway, much of which predates the 1956 introduction of the Interstate Highway System. One of the busiest highways in the country (and state of Pennsylvania) and with the road being highly over capacity, it has become notorious for its chronic congestion, especially during rush hour. Rush hour in Philadelphia is usually as early as 6 am, with many in the Delaware Valley using the Schuylkill to reach Central Philadelphia and some of Philadelphia's western suburbs. The rugged terrain, limited riverfront space covered by the route and narrow spans of bridges passing over the highway have largely stymied later attempts to upgrade or widen the highway. An average 163,000 vehicles use the road daily in Philadelphia County, and an average of 109,000 use the highway in Montgomery County. Its narrow lane and left shoulder configuration, left lane entrances and exits (nicknamed "merge or die"), common construction activity and generally congested conditions have led to many accidents, critical injuries and fatalities, leading to the highway's humorous nickname of the "Surekill Expressway" or in further embellishment, "Surekill Distressway".