As above a credit card and a £20 note are your best friends when disaster strikes. I got knocked off my bike, albeit at slow speed a few years back and picking the bike up I thought it (and me) were okay. Unfortunately neither was the case. The bottom bracket shell was cracked right through and I had a light concussion and two broken ribs.
When eventually the police came the driver had long since scarpered and my wife was on a 7hr transatlantic flight going in the wrong direction. I had to admit defeat and bung the whole lot in the back of a taxi after a visit to A&E.
I've heard tall tales of my grandad whittling a new rear drop out from a piece of oak he found at the side of the road and replacing the broken one for a 20 miles ride home. In the Highlands, in February. The reality was less romantic, it broke again after 500 yards and he had to walk the five hours over hill and Dale to the youth hostel, in the rain!
£20 saved my bacon on several occasions. Better, and a lot lighter than any fancy 30 part multi tool