According to U.S. Transportation Secretary Ray LaHood, and representatives from nearly all 50 states, the mandated 2018 sign swapping would've cost millions of dollars that didn't need to be spent. Local governments can better determine when a sign needs to be changed, and it makes more sense for the move to be handled this way then by a random point in the future. It also saves everyone a lot of dough.
The state of Minnesota would need to spend anywhere from $55 to $75 million to hit the 2018 deadline, for example. Delaware would fork over $60 million, while the New York City on its own would spend millions of dollars and require 12 to 16 years just to complete the project. That would put the city while behind the 2018 deadline.
The federal government heard the cries, and it's listened. 46 of the proposed deadlines have been eliminated. 12, however are remaining active because they are deemed crucial to public safety. Examples of this important intersections include railroad crossings without flashing lights and one-way signs wherever necessary.