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The Momentum Papers · 03 of 15 · The physics of loss

The tax no government collects

Every time a road forces a vehicle to stop, it destroys energy that was already paid for, then demands it be bought again at 12 to 20% engine efficiency. This is the second energy tax, and it never appears on any bill.

The engineering reality

What a moving vehicle carries

Vehicle efficiency is normally assessed at the engine, under test conditions across urban, extra-urban and motorway cycles. That approach misses the most wasteful part of a real journey: the places where momentum is repeatedly destroyed by managed stops.

A moving vehicle carries two packages of kinetic energy, and both are destroyed at every forced stop.

Linear kinetic energyKE = ½ m v²The forward motion of the vehicle and its load, where m is mass and v is speed.
Rotational kinetic energyKE = ½ I ω²The spin of wheels, tyres, axles and drivetrain, where I is moment of inertia and ω is angular speed.
Total destroyed at every stopKE = ½ m v² + ½ I ω²Both packages are lost at once, and both must be bought back from standstill.
Up to 3×The energy needed to get back to speed, versus the energy lost stopping
12–20%Engine efficiency when pulling away, its worst band
~14 GWhDestroyed a year at one busy junction carrying 18,000 vehicles a day

Why the loss is greater than it looks

Eight reasons stop-start compounds

01 Static inertia

From rest, extra torque is needed before any progress is made, at the engine’s least efficient point.

02 Full drivetrain reset

Wheels, discs, hubs, shafts and differentials all have to be spun up from zero.

03 Gear change cascade

Rotational inertia is rebuilt through every gear on the way back up to speed.

04 Brake hold

Electronic brake hold adds resistance at pull-away that the engine must overcome.

05 Turbo rebuild

Boost pressure collapses at each stop and has to build again from nothing.

06 Rich mixture

Engines run richer fuel mixtures when pulling away to avoid stalling.

07 Tyre deformation

Rolling resistance is highest at very low speeds, exactly when efficiency is already worst.

08 Gradients

On a slope, every one of these is amplified while the engine also fights gravity.

The missing measure

Europe has engine standards. It has no road standards.

European emissions policy has raised the bar for engines since Euro 1 in 1992. Each step has cleaned the exhaust. But it treats an infrastructure problem with an engine solution.

A real infrastructure efficiency standard would measure how much of the energy put into a road network is kept as useful forward motion, and how much is destroyed by forced stopping and restarting. No such standard exists. No regulator asks for it. No government measures it.

However efficient an engine becomes, if the road network is designed to destroy momentum, much of that efficiency is wasted before it does useful work.