Jormungandr, the Snakebot of Doom, has left the rubble of New York from where it has just finished destroying all the world's petroleum refineries and metal smelters, and has just made a right-hand turn in the New York suburb of Mount Vernon that puts it on course for Boston. As it has coiled itself into a billion-and-a-half ton hoop and is currently rolling toward Boston at around 75 kilometres per hour, Bostonians have three and a half to four hours before it arrives, and presumably begins steamrolling the city flat, much as it did to New York.
If Bostonians were to be informed by the Emergency Broadcast System within minutes of Jormungandr's setting course for their city, how many could be expected to escape from a nearly 8km-wide swath of total destruction across the most built-up part of Boston? They have three and a half to four hours before Jormungandr arrives, plus whatever time it takes to reach their location at its city-destroying speed of about 5 KPH.
Jormungandr is really only going to focus on steamrolling buildings 3-4 stories or taller that are among other such buildings, or individual buildings of 6 stories or taller unless they are a long way from other target buildings. Buildings 24 stories or above will be targeted regardless.
More importantly, how can I work out for myself how many people might be evacuated from a given area of a city, given an amount of time to conduct the evacuation?
Jormungandr is probably going to flatten a swathe about 7.5km wide from Roxbury in the south to Revere in the north, including the University of Massachusetts and Boston Logan International Airport in the east (but not the runways, and maybe not the terminal buildings either), to Harvard University in the west, that area containing most of the large buildings. It will also include a 400m wide track coming in from the south-west, and leaving to the north-west. However, Jormungandr has not announced these plans. All that is known is that it appears to want to flatten large buildings.
For my Juggernaut stories so far, look at https://medium.com/@MontyWild. Chapter 5 describes Jormungandr's attack on New York City, which would likely be much like its attack on Boston.