I always thought these were displayed after a fatality?

It's just that on my way home yesterday i saw what looked like a fairly innocuous bump and today the signs were up??

Have you called the police then? They can probably tell you when they use them while you give your wittness account.

Maybe it wasn't as innocuous as it looked. How many police were there when you went by?

old people and kids are quite small

I suspect that they'll put them up if they dont think they have a decent grasp on the events of that day, rather than limiting it to fatalities?

I didn't actaully witness the crash but just the aftermath and the police were at the scene

Well there might have been a pedestrian in the middle of it all, or perhaps another vehicle involved in the accident left the scene. But maybe just the stories of the people involves aren't matching up, or quite strong accusation is being made. It could be a simple as trying to confirm which of the occupants was actually driving. Perhaps the pissed owner of the car is claiming that his sober passenger was at the wheel.

also put up when someone leaves the scene of an accident