If two motorcades carrying prime ministers meet at an intersection, which one gets priority?

Right now, we've got a boatload (well, for Boston) of foreign dignitaries flitting around, leading to all sorts of motorcade spottings and resulting traffic tieups all over the place, from Harvard Square to the Bank of America building downtown. The prime minister of Vietnam is in town. So's the prime minister of Ireland. The speaker of the House of Commons in the UK is about as well - as is our former speaker of the house, who granted, probably doesn't get flags on the cars in his motorcade, but still.