But what's the real cost of making seatbelts mandatory? The government could still make that the law without direct "mandatory seatbelt" slush pool funding. That would come from a proposal for a "Public Safety Think Tank" or something - a place where elected government officials would still have discretion over public issues.
On a local level, it could be more impactful: locals could vote if they want the rickety bridge rebuilt or if they want a new fleet of cop cars purchased for the state patrol.
Seatbelts are a bad example from that perspective of course. We'd have to look at the financing of things most don't have a benefit from, but a minority really really needs.
On a local level it's something different, and I'm all for it. I generally think more of the decision making has to be put into the hands of communities, and state/federal governments should only be responsible for what's required to connect them, or where there is no community to care.
//edit: We even had a project similar to that on Horizon, for a candidate for the London mayor. The plan was to mirror the cities funds with assets, with the chance to let the public vote on some transactions later. His campaign wasn't successful, and the project itself never got off the ground, but it shows that the thought isn't very far fetched :)
http://mayorschain.com/