Quantitative limitations on anti-terrorism measures?

The Economist this week has an article on characteristics of the (UK) Labour government's lawmaking as a response to terrorist threats. One the features mentioned: repeated assurances from government and police forces that new powers will be used with moderation. The paper writes:

"It is now routine for the attorney-general or the director of public prosecutions to promise that a sweeping new power will not be used in a sweeping way. Such pledges were made for Anti-Social Behaviour Orders and control orders. this week, Mr Blair insisted that only a "very small"number of suspects would be held without charge for up to 90 days. (...) Such promises tend to be kept - for a while."

The article proceeds with a description of how the new measure of 'control orders' has been deployed "against protesters, beggars, prostitutes and the mentally ill". This phenomenon of increased use of powers once granted is obviously not limited to the United Kingdom.

Governmental credibility is a serious problem here. Even those in parliament and public who would generally approve of stricter measures for severe cases, will be reluctant to offer their support of broad powers with unclear qualifications. Generally, independence for key-decisionmakers is a way out of credibility conundrums (look at central banks) and here too, independent oversight - by the judiciary - should be the preferred solution.

For the special case of measures applicable before any judicial oversight becomes available, such as the proposed 90-day detention before charges now defeated in the House of Commons, a specific credibility enhancing solution could be envisaged in the form of quantitative limitations on special powers. Would it not be possible to allocate a certain number of pre-charge detainment rights to police authorities, for use in cases they select? Once these rights would have been exhausted, only detention under the standard regime would remain possible. I realize it would be important to implement such a system without the possiblity of terrorists knowing in advance whether or not sufficicient rights remain for their detention, but this is a practical problem that could be overcome. Anyway, this may be an utterly foolish idea so I'm looking forward to comments.