Instead of a bomb how about a warning that these vans exist? You could have a bunch of nerds leaving arduinos in Ubers around NYC. Have the arduinos measuring radiation and phoning home with any locations that are above average. At the very least it would be an interesting map to look at
You're a much more constructive person than I! It would be interesting. Hook it up to a 360 panoramic camera to capture the source as well.
I don't know if this is possible, but I suspect it may be.
You may have stumbled onto a very dangerous idea... not one that could be used for good.
Why would this be any more dangerous than any number of other triggers?
Probably because the trigger would be sensitive to such low amounts of radiation that many sources could set it off.
I would imagine the sort of people to use such a trigger wouldn't care.