umpireal

The momentum tax, measured

Two roads,
one journey.

Between Kells and Clonee, drivers choose between the M3 motorway and the R147, the old N3. Same start, same finish, almost the same distance. One flows. One stops and starts 28 times. This model shows what every stop costs.

A natural experiment

Most claims about congestion compare different roads, speeds and distances. These two routes remove almost every difference except one: how often you have to stop.

Toll plaza (M3) Each orange dot is one stop or slow-down: lights, roundabouts, ramps or speed limit changes
51 kmM3 route, Derver to Clonee, 2 toll plazas
52 kmR147 route, through Kells, Navan and Dunshaughlin
~1 in 3cars in the corridor use the R147 (TII counts, 2025)

One journey, four ways

To isolate the cost of stopping, every option below is driven at the same cruising speed. Any difference you see comes from stop, idle and go, nothing else.

Why the tag lane doesn't fix it

A vehicle's motion energy rises with the square of its speed. So slowing down, not just stopping, throws most of it away. Drag the slider to choose how slow a vehicle goes through a toll lane from 120 km/h.

From barriers to free-flow tolling

Free-flow tolling uses cameras and tags so nobody slows down at all. Ireland already runs it on the M50 through eFlow. The toll is still paid; the momentum tax is not.

Based on 2025 TII counts beside both plazas: about 24,900 vehicles a day at Blackbull and 14,400 at Grange. The tag share is not published, so it is left for you to set.

The corridor in a year

Now scale one journey up to every vehicle TII counted in 2025. Cars and vans are counted separately from HGVs, because a heavy truck loses far more at each stop. Two losses are kept apart: stopping on the R147, which free-flow junctions would prevent, and slowing or stopping at the M3 toll plazas, which free-flow tolling would prevent. Part 2 shows a whole trip. Part 3 counts only the waste from slowing, stopping, idling and pulling away: not the fuel and time it takes to drive the road itself.

1Who uses each road, every day

2One whole trip, Derver to Clonee

The same vehicle on each road for the whole trip: driving the distance, stopping, tolls and time. Both roads are driven at the same speed so the comparison is fair; in real traffic the R147 is slower still, so its time cost is higher than shown. Fuel price and value of time are set in the journey section; tag share in the free-flow tolling section.

Savings compare each M3 option with the R147. A negative saving means that option costs more for this item.

3Every year, all traffic on each road

Each road's own traffic, as counted by TII in 2025, before and after a fix. This counts the waste from stop-start only: the fuel and time lost slowing, stopping, idling and pulling away. The fuel and time to drive each road are not included, because a fix does not change them. On the R147, the fix is free-flow junctions on the open road between towns: Kells, Navan and Dunshaughlin town-centre stops, ramps and speed limits stay. On the M3, it is camera tolling.

Detail by stretch and plaza, cars and HGVs separately

Fuel lost a year. R147 at its posted 80 km/h on open road and 50 in towns; M3 at 120 for cars and 80 for HGVs, with the tag share set in the free-flow tolling section. HGVs are split into rigid trucks (6 t) and articulated trucks (26 t, an average load) using TII's vehicle class counts; the one-journey section uses a fully laden 40 t truck instead. Vans are counted with cars, and the R147 Navan figures use the Tara count, so treat each stretch as an estimate.

Who gains when the stops go

Drivers

Less fuel, less time and less wear on every trip, whichever road they choose.

The State

The M3 contract, which runs to 2052, includes traffic guarantee payments when use falls short. Every driver drawn back to the motorway reduces them.

The towns

Kells, Navan and Dunshaughlin carry through-traffic that has a motorway beside it. Fewer stops mean quieter, safer streets.

The climate

Fuel that is never burned needs no production, shipping or offsetting. Prevented energy is delivered at 100% efficiency.

Two fixes, and the stops they can't reach

Free-flow tolling

Removes the plaza stop. Cameras and tags, as on the M50.

Fair pricing

Removes the reason to divert. A toll set below the true cost of the alternative brings traffic back to the motorway.

Neither fix touches the R147's own junctions, where traffic stops because other traffic crosses it. That is the momentum tax umpireal is built to prevent: a free-flow junction where no stream of traffic crosses another, with AI compute and battery storage in the rooms within the raised road paying for it. See how it works.

Method, assumptions and sources

How the physics works

Each stop or slow-down is costed with the Umpireal Crossing Energy Loss Model (EU v5): the fuel needed to rebuild the vehicle's motion energy, its rotating parts, rolling resistance and air resistance during acceleration, at the engine's efficiency for that manoeuvre, minus the fuel it would have used cruising the same distance. Idling at stops is added at each vehicle's idle fuel rate. Time lost is the braking, waiting and acceleration time minus the time to cruise the same distance.

Key assumptions
  • One journey: car 1,800 kg petrol, 0.8 L/h idling; HGV a fully laden 40,000 kg articulated diesel, 2.4 L/h idling, about 32 L/100 km cruising. The year: HGVs split into rigid (6 t) and articulated (26 t average load) trucks using TII vehicle class counts at each site.
  • One journey: car at 90 km/h and HGV at 80 km/h on both routes, so only stopping differs. The year: R147 open road at its posted 80 km/h, M3 at 120 km/h for cars and 80 km/h for HGVs.
  • R147 events from a local drive survey: 6 stops in Navan; 3 stops and a roundabout in Kells; Johnstown lights; 2 roundabouts at Lismullen; 3 to 4 stops in Dunshaughlin; 2 ramps; the Rathbeggan crossing (assumed to stop half the time); 6 further roundabouts; and town speed limit changes.
  • Idle: 20 seconds at most stops, 30 at Johnstown, 20 to pay at a barrier. Peak queues are longer, so these are conservative.
  • Each R147 section's figure assumes every counted vehicle passes all of that section's stops. Some turn off, so each section is an upper estimate for that section.
  • Tank-to-wheel CO₂: petrol 2.31 kg/L, diesel 2.68 kg/L. Well-to-wheel adds 19%. Energy at 9.25 kWh/L. Wind comparison at a 30% capacity factor.
  • Tolls: €1.80 per plaza for a car, €4.40 for an HGV with trailer (Eurolink).
Sources
  • Transport Infrastructure Ireland, traffic counters 2025: M3 Jn4 to Jn5, Jn5 to Jn6, Jn6 to Jn7, Jn9 to Jn10, Jn10 to Jn11; R147 at Martry, Tara and between Blackbull and Dunshaughlin.
  • Route distances and times: Google Maps, Derver to Clonee.
  • Eurolink M3 toll charges.
  • Umpireal Crossing Energy Loss Model EU v5. Canonical figures: umpireal.com/facts.

All figures are modelled estimates, rounded, and marked approximate where shown with ~.