I was once stuck in NYC's Holland Tunnel in a traffic jam behind a mid-tunnel car accident. With both lanes of cars backed up, there was no way to get a tow truck through in the direction of travel; they eventually sent one in from the other end, driving backwards, to clear the mess.
In the UK, they've got at least one solution for rapidly getting a tow vehicle on-site--and it's a motorcycle. Since the cars driven in the UK are generally smaller than what Americans drive, it's a viable option. Check out their modified Honda Goldwing "Retriever" model:
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Incredibly, Autoblog reports that the thing can tow 5,500 pounds!