Yeah!!!

And then the deer that eat the roadside plants would glow in the dark and hunters could pick 'em off easier.

If that's a problem it might be possible to transform plants inedible to animals.

it's interesting that this appears in the same window with 'paint that stands out'--I think flourescent paint would be great on a car.

As for the highway weeds, unless it was a beneficial mutation for the plants (and I can't see how it would be) I think it would disappear in a short time. Also, I think the intensity of bio-luminescent things is really low compared to your headlights. Probably wouldn't even be able to tell they were glowing unless you look way-out.

Yeah, you probably woudn't be able to see them glow unless you turned off your headlights, which would be a bad idea. If you could see them, there's still the problem that they might grow in the wrong places and confuse drivers. They might even lead drivers off the road! I think it would be safer to mark the road edges with artificial markers as they already do. That way, you know the markers will be where they're suppose to be.