Terrorism Weekly: What is Behind Britain's Transportation Alerts?
This is undoubtedly a reminder to the travelling public to be vigilant (although authorities have to be careful: frequent alerts arguably have the opposite effect). However, there is another audience for this alert. In this case the threat is posed not by known terrorists under surveillance --- about 2000, according to the British intelligence service MI5 --- but the terrorists who are, in the parlance of Donald Rumsfeld, the “known unknowns". It is a small group or even an individual who has managed to remain under the radar that create concern for obvious reasons.
The message to them is “we are on to you”. Although some terrorists may not be terribly bright, they generally tend to be rational and thus can be deterred from their plans by a superior show of force or greater vigilance.
Or so the state may hope.