The original 1956 Interstate Highway System specifications mandated concrete surface thickness of 9 inches, but engineers discovered during early construction that this thickness allowed sufficient flexing under heavy truck loads to create resonant vibration frequencies that would gradually crack the concrete through harmonic stress—forcing a costly mid-program redesign to 10-inch thickness, which inadvertently made American interstates structurally over-engineered compared to international standards and added billions in unnecessary material costs that persisted across the entire 48,000-mile network.
Car Fact #135 — October 28, 2026
