In Atlanta, there's a truck that has a video screen installed on the back. I'm not totally sure what it usually plays on the video screen, since I wasn't close to it long enough to make it out. However, I know that it's incredibly distracting.
And yet, I have a feeling that if you could determine the rate of accidents around the video truck, it would be lower than the rate of accidents everywhere else. Because when people see the truck, they not only get distracted, but they become aware of getting distracted.
It's such a shock to the system to drive by a truck with an animated sign on it that it forces a nearby driver to pay attention. This is great, but it leads one to a false conclusion: video trucks make drivers safer.
In reality, the video truck only has a net positive impact on safety if it is rare. If more video trucks were on the road, their rarity (which was the thing that made them cause people to pay attention) would decrease, and they would have a net negative impact on safety, as expected.
Typically, if something is good for you, doing more of it gives you more good, albeit at a reduced rate (most things have decreasing marginal utility). But there is a class of things where while a little is good, a lot is really, really bad. In fact, most things are in this class (food, water, money?), and it may be smart to suspect that everything is in this class. That is, there is no such thing as an unalloyed good. Getting more of anything can become a curse, even (and perhaps especially) if you've devoted your life to getting it.
"An idea starts to be interesting when you get scared of taking it to its logical conclusion." -- The Bed of Procrustes, Taleb