So it's stupid to think/hope that someone might drive a little more carefully around a car with a pregnant woman or a young child in it? Or perhaps just be a bit more aware that the driver displaying the sign could be distracted or worried more easily and so it's a car to keep your distance from?
But it's not stupid to think a "Little Tarquin on Board" sign is an advertisement to hoards of mobile perverts?
I've already given my reasons for thinking these signs are a good thing, but now I wonder why are they seen so different from the ones they display on the motorway reminding people to take a break on a long drive, or the flashing ones at roadsides that tell you the speed you are doing, or the Kill your Speed, Not a Child ones near schools?
If nobody looks at M-T-B on Board or Baby on Board signs and thinks "Okay, I'll crash into a different car" then why do the highways agency or whoever still think it is worth reminding people to take care when tired or to slow down, things every driver has to learn to even pass the test? Surely by this reasoning ("I'll crash into a different car then") nobody pays attention to them either or thinks "Okay, I'll go crash outside the bank and not the school then."
Those things are static, constant things every driver should already know and do without a reminder. The other cars on the road are an unknown quantity and anything that can flag up a potential risk or problem should surely be a better thing, not a thing to be smirked at.