Start the Chapter "Route Kanal" and get to the point where the Metrocops attempt to ambush you at the bridge (this is after the station 8 raid, where you find the combine encampment). Preserve all of the physics objects throughout the fight, and attempt to NOT destroy the bridge. There should be a small rampish thing next to the sewer grates by the metrocop that jumped onto the car. Stack up alot of physics objects there, and you can escape the level! using this, you can check out route kanal from the top, or drop down into the previous level.
Found by ArmoredScout