Agree with firefly bob.
I've been in the rail industry for 15 years and random D & A screening was introduced just after I joined. Everybody in a safety critical role is subject to screening, that includes signalling designers, managers and other office based jobs. On the railway each company has to randomly screen a certain percentage of their staff per year. Generally you get 24 hours notice as you have to report to a medical centre. Anyone who fails a breath test when given 24 hours notice deserves to lose their job.
You do not get any notice of a test if there is an incident. Very often everyone in the area will get tested following an incident. A signalman could easily be tested as a result of a train driver running past a red signal. We all go to work with the knowledge that we could get tested the moment we walk through the door and therefore most of us make sure that we are comfortable that we have left sufficient time since our last drink.
As far as aviation goes the situation at the moment is that any pilot could be screened at any time if someone suspects they may be under the influence of drugs or alcohol. I would have thought that the knowledge that screening is possible will make people think twice before risking an extra drink and will therefore protect them from failing a breath test should they be unlucky enough to be involved in an incident whilst taxying out.
If the thought of a breath test worries you then you probably should have had 1 pint less the night before. And for the record, I do like a drink myself as do most railwayman!
SW