In reality, many of the barricades weren't physically inescapable the way Hugo describes this one. By the time the barricade closing the Faubourg Saint-Antoine was taken, a lot of the insurgents decided to flee from the smaller ones using whatever means was available to them. In some cases this might mean an alleyway that had been left open to use as an escape route or for messengers from other barricades, or yes, traveling across rooftops as Amarguerite suggested in Some Friendlier Sky.
The biggest difference between Hugo's representation and actual history is that in actual history the guardsmen took prisoners. When they overtook the barricades they killed anyone who defended themselves, but a surrender meant being taken captive. (Then you could later plea "oh, no, sorry, I got caught on the wrong side of the barricade!" which apparently was common and effective. Most of the insurgents taken captive alongside Charles Jeanne, for example, were eventually released.)
Not that Hugo specifically says "they killed everyone in cold blood!" since Bahorel certainly wasn't about to surrender in the moments before they shanked him with bayonets. But had some of them surrendered, they probably would have lived.
(And don't forget that after the barricades were all taken care of, the National Guard started to conduct house-to-house searches to see if they could round up any more escaped would-be revolutionaries. So once you've physically made it away from the barricades, you'd best find yourself an alibi and/or a hiding spot.)
"Quand vous aurez besoin de Bahorel, capitaine, Bahorel est là! Je sais faire trébucher tous les chevaux du garde-corps avec une ficelle... Rien qu'une petite ficelle. Enfin, pensez à Bahorel du Café Musain!"