When an occasion is so popular/awesome that it gets crowded out with people so much that cars cannot drive down the road! Usually a sign of good times and banging music.
Festivals, Carnivals, & live gigs are usually them main culprits
'Goin up to carnival can you drop me off in Notting Hill bredrin?'
'Get the tube man, its gonna be a ROADBLOCK!'
'Dr. Dre doing a live free set in the ASDA carpark, get down there early, its gonna be a ROADBLOCK!'