What happened
A team modeled electricity and transportation networks in a real US city to see what happens when one fails. It turns out you can quantify how much damage spreads from one critical system to the rest — power goes down, traffic can't flow; roads get damaged, power distribution breaks.
Why it matters
Cities treat electricity, water, roads, and sewage as separate problems. They aren't. This paper shows you can actually measure the cascade: knock out a transformer station and you've crippled traffic signals, hospitals, water pumps, everything else downstream. Until now, city planners had no way to test these interdependencies before something actually broke. This matters because most disruption happens this way — not from one catastrophic failure, but from one system's collapse triggering failures in three others. Cities can now model which points of failure are actually dangerous.