How much time is enough time to warn of a bombing? Any warning at all might seem a world away from the surprise of the 9/11 attacks, but in Northern Ireland, it's the custom for IRA terrorists to alert authorities, allowing evacuation of an area before detonating explosives. On Tuesday, though, a police chief in Northern Ireland struck out at IRA members for notifying him under 20 minutes before an explosion.

"So what would constitute fair warning?" asks AOL News' Theunis Bates. One mine clearance expert says that for a bomb the size of the one in question--a car bomb driven into a set of gates--it's standard procedure to evacuate everyone within a 200-meter radius. In a densely crowded area, and with a limited number of policemen on hand, that can take a lot of time.

But as Bates notes, the same expert also adds that, "when it comes to bombs in public spaces, there's no such thing as a fair warning. 'Police and bomb disposal teams are never going to get enough time ... You simply can't put a figure on it.'"