Robert Heinlein's 1940 story The Roads Must Roll imagined continuous belts stretching between cities at 100 mph. Airport walkways top out around 3 km/h and 300 m long. Let's push six orders of magnitude harder: a 20 km commuter belt from suburb to downtown, 3 m wide, running at 20 m/s (72 km/h).

The speed-matching problem. You can't step from stationary ground onto a belt moving faster than a sprinter. Heinlein's solution: nested parallel strips at graduated speeds. Use six lanes at 5, 10, 15, 20, 30, and 50 km/h, plus a 72 km/h express. Each transition is ~1.4 m/s — brisk but survivable, roughly the shear a moving-airport-walkway rider handles today. Handrails on each strip run at the strip's own velocity so you have something to grab.

Belt mass and inertia. A steel-reinforced elastomer belt 5 mm thick, 3 m wide, has linear density around 118 kg/m. A 20 km loop (40 km of belt) weighs 4.7 × 10⁶ kg. At 20 m/s the belt's kinetic energy is:

KE = ½ × 4.7×10⁶ × 20² = 9.4 × 10⁸ J ≈ 260 kWh

That's the startup energy — about $30 of electricity, but concentrated in a single belt. If that belt snaps, you're releasing the kinetic energy of a fully-loaded 747 hitting a wall.

Steady-state power. Rolling friction on well-designed idler bearings runs ~1% of normal load. For the belt alone:

P = μ × m × g × v = 0.01 × 118 × 9.8 × 20 = 231 W/m
Total (20 km) ≈ 4.6 MW

Add passenger load. At crush density (2 people/m²) the express strip carries ~6 riders/m × 75 kg = 450 kg/m, pushing friction losses to ~13 MW. Trivial compared to a subway line's traction power.

Throughput annihilates rail. One 3 m express strip at 20 m/s and 2 people/m² moves 216,000 passengers per hour per direction. NYC's busiest subway line peaks around 40,000. Because there are no stations — you just step sideways to slower strips near your stop — dwell time vanishes.

The failure modes get spicy.

• Belt fracture: 260 kWh released as whip and heat. Segment the loop into 500 m sections with clutched couplings; a break dumps only ~6 kWh — survivable.
• Drive redundancy: Distribute linear induction motors every 100 m. Lose 10% and the belt just decelerates.
• Passenger stumbles: Someone falling on the 72 km/h strip experiences the same road rash as a motorcycle low-side. Full-height dividers between strips and mandatory seated pods on the express solve this — which is exactly what Heinlein described.
• Thermal expansion: A 20 km steel-cored belt expanding 12 ppm/°C stretches 4.8 m over a 20°C day. Requires active tensioning takeup towers with hundreds of tons of counterweight.

Why nobody's built it. Capital cost per km is comparable to elevated rail (~$200M/km) but the belt has a 15-year fatigue life versus 50+ for rails. A subway wears out in wheelbases; a rolling road wears out everywhere at once. The maintenance shutdown problem is what killed every real proposal — you can't service a continuous belt without stopping the entire city commute.