The London Marathon has a refuse team at the back of the run which appears to be comprised of counsil wrokers, volunteers and private companies. After all the runners have left the start it follows them and collects the vast majority of the rubbish and barriers (on a whopping great convoy of lorries).
I was volunteering with cadets at mile 8 this year and basically what happens is when there are so few runners that the streets are nearly clear, you sweep up all the bottles etc. Into bags and then they collect it all. Chaos ensues shortly after as cars, vans and HGVs try to get through one lane streets in a long convoy avoiding rogue children and volunteers all over the street.
Of course the larger distance and smaller numbers of partipants in cycling events make this method way less suitable.