Adding a road can worsen traffic at Nash equilibrium
Braess's paradox (1968): in a congested network with Wardrop equilibrium (no driver can unilaterally reduce travel time), adding a zero-cost link can paradoxically increase everyone's travel time. The classic example uses latency functions t = x/100 (flow-dependent) and t = 45 (constant), where the new link causes equilibrium travel time to jump from 65 to 80 minutes.