Boston was in lockdown Friday. The machinery of a major metropolitan area in the richest nation on earth had come to a grinding halt. We know why this is happened – a terrorist manhunt – but how, exactly, does a modern bustling city come to a full stop?

In fact, much of ordinary life continues. Water still comes from the taps for a shower; you can telephone your family and friends; you can even work on your computer or read quietly in the backyard. But one key aspect of city life stopped: the movement of people. What matters most in a lockdown of this scale is the ability to halt the circulation of people.

Whether or not a lockdown works often depends on who – the citizens or the terrorist suspect – can stay still the longest.

The prevailing idea of the lockdown is “shelter in place” – an instruction to remain precisely where you are. The phrase came into general use in the 1970s, in response to planning for population protection in the event of nuclear attack – often with a specific concern that a chemical, biological or nuclear device might have contaminated an area.

Terrorism is our new nuclear threat and the phrase has recently been applied in planning scenarios similar to the reality that unfolded in Boston Friday. As with the operation of public transportation on a normal day, such directives require the cooperation and goodwill of the population at large.