The slippery slope of federal funding

There is a motorism-based ideology in transportation planning that says that the public sector should never pay for internal circulation. Like most of the tenants of motorism, it's based on early interstate highway development, and the ideology of what the Feds were and were not going to pay for. 

The Federal government was going to pay for the limited access highways, and the state governments were going to pay for anything local. But, given the generosity of Federal funding for interstate construction, the definition of 'local' shortly ceased to include on-ramps and off-ramps, or viaducts onto the freeway, or access ramps to the viaducts, or anything that could be reasonable be justified as being part of an Interstate so the feds paid for its construction/paving.

But interstates come with interstate design standards, which include high speeds, which require large curve radii, which conflicts badly with urban street grids, necessitating a lot of demolition. And once you've got someone willing to pay for demolition, it's very tempting to direct that energy to someplace you've been wanting to do slum clearance anyway.