How is an ID check going to stop a suicide bomber?
Well, lets take 100 potential suicide bombers, or criminals or other malcontents. Scenario 1, we do nothing. Don't look at them, no checks, free to carry onto a bus or metro anything at all at anytime they please, even during a time of heightened alert.
Scenario 2, we don't do nothing. We look at them, look for suspicious behaviour, profile potential threats, check some ID's.
Sinmple question. Which scenario is more likely to deter or detect someone with bad intentions?
If it is known that on a certain bus all ID's are checked, or they are checked at random, would you be more likely to use this bus, or one where you knew nothing was being done?