I've seen a couple of ads trying to get people to stop texting while driving by responding with "#X". It's meant to be a shorthand to let people know you're driving and so won't be responding to texts for a while. However, there are a few problems.
First, unless you know what "#X" means, if you get a text like that, it's not going to make sense, and the problem persists. In the ads, a person receives "#X", pronounces the person responsible and proceeds with their day in a good mood. The person who doesn't receive the "#X", however, keeps texting, being ignored and growing more furious. If you don't understand it, however, the "#X" won't solve that problem. It needs to be something you discuss with people.
Second, why "#X" at all? The ads all show people about to start driving, receiving a text and responding with the tag. But if you're not driving *yet*, why do we need a code? Just text "Driving. Can't respond." or something similar. Same effect. The only reason to make "#X" a standard is to be a quick response you can dash off *WHILE DRIVING* which defeats the whole purpose.
Third, if you really want this to be a thing, what you need is for the phone OS to change. Monitor the phone's physical speed via GPS. If it's over a certain speed, there will be no text alerts and, optionally, you can have it respond automatically with "Driving. Can't respond." No need for an ad campaign or a special tag, just a more considerate phone OS or a Don't-Text-And-Drive app.
Mokalus of Borg
PS - I'm fairly sure I'm not the only one who's had that idea.
PPS - In fact, here's one you can use now.