Almost everyone believes a taxi meter charges for distance, and adds a waiting charge when the car is stopped. It is a reasonable guess. It is also wrong, and the difference shows up as money on every ride that hits traffic.
Understanding the real mechanism takes about five minutes and changes how you read a fare for good.
The rates on the card
A tariff is usually three or four numbers:
| Term | Also called | What it is |
|---|---|---|
| Flag-fall | base, minimum, hire charge | Charged the moment the ride starts |
| Per kilometre | distance rate, running rate | The distance charge |
| Per minute | waiting rate, time rate | The time charge |
| Tick | unit, increment | What the display jumps by |
Many tariffs bundle an opening allowance into the flag-fall — 100 for the first kilometre, 80 a kilometre after is the same thing as a flag-fall of 100 that covers the first 1,000 metres. Some also include an opening time allowance.
The part that is not obvious
Here is the rule that surprises people:
A meter does not add the distance charge to the time charge. For every moment of the ride, it charges whichever of the two is worth more, and never both.
Internally there is a single accumulator. Between one instant and the next, the meter works out what that interval would be worth at the distance rate, what it would be worth at the time rate, and adds the larger. Not the sum. The maximum.
This is why "waiting charge" is a slightly misleading name. The time rate is not something that switches on when the car stops — it is running the whole time, and it simply loses to the distance rate whenever you are moving quickly enough. Standing still is not a special case; it is the limiting case of slow.
Why summing would be a disaster
Take a tariff of 80 per kilometre and 20 per minute, and one minute of driving at 40 km/h — 667 metres covered:
- At the distance rate: 0.667 km × 80 = 53.30
- At the time rate: 1 min × 20 = 20.00
The meter charges 53.30. A meter that summed would charge 73.30 — 37% more, for the same minute.
And it would get worse the slower you went, because a slow kilometre takes more minutes to cover, so the time charge climbs while the distance charge stays the same. Summing double-charges precisely the part of the journey where a taxi already spends most of its money: the congested part.
The third number nobody prints
Set a per-kilometre rate and a per-minute rate and you have set a third value whether or not you noticed: the speed at which the two are worth the same.
changeover (km/h) = 60 × per-minute ÷ per-kilometre
With 80/km and 20/min, that is 60 × 20 ÷ 80 = 15 km/h.
Above 15 km/h, the distance rate is winning and you are being charged for kilometres. Below it — crawling, queuing, stopped at a light — the time rate is winning and you are being charged for minutes. It is a smooth handover, not a switch: at exactly 15 km/h both come to the same number, so nothing jumps.
Two things worth knowing about it:
The tick has no effect on it. It falls out of the two rates alone.
It is the single most important number in the tariff for a city ride, and it appears on no rate card anywhere. Set the time rate too low and an hour of gridlock is nearly free. Set it too high and a red light costs more than the kilometre that preceded it.
Try it against your own city's rates. If the changeover comes out at 3 km/h, that tariff barely charges for waiting at all. If it comes out at 30 km/h, almost your entire urban journey is being billed as time.
Why the number jumps rather than glides
The tick is why a meter visibly steps — 5, then 10, then 15 — instead of sliding smoothly upward.
The accumulator runs continuously, but the display only shows completed units. A part-unit in progress is not charged yet. So the fare only ever takes tick-multiple values, and it jumps at the moment the accumulator crosses one.
This is worth knowing for a practical reason: a meter that glides is not modelling a meter. It is running a smooth total and rounding it for display, which is a different calculation.
Putting one ride together
Tariff: flag-fall 100 covering the first kilometre, 80/km, 20/min, tick 5. Changeover 15 km/h.
| Stretch | What happens | Accrues |
|---|---|---|
| First 1.0 km | Covered by the flag-fall | — |
| 3.0 km open road, ~40 km/h | Distance rate wins | 3.0 × 80 = 240 |
| 6 min in traffic, ~6 km/h | Time rate wins | 6 × 20 = 120 |
| 2 min stopped at lights | Time rate, no special case | 2 × 20 = 40 |
| Final 0.8 km | Distance rate wins | 0.8 × 80 = 64 |
Accrued 464, floored to the 5 tick = 460, plus the flag-fall of 100 = 560.
Note the 600 metres covered during those six traffic minutes contributed
nothing on top of the 120. That interval was charged once, on time, because
time was worth more. That is the max rule doing its job.
Extras, and the order they apply in
Night surcharges, airport pickups, luggage, tolls and booking fees sit outside the accumulator. They are added at the end — and specifically after any minimum fare, not before. A minimum fare is a floor on the metered portion, not on the total bill.
What to do with this
Two things, next time you take a metered ride:
Work out your city's changeover speed. Sixty times the per-minute rate, divided by the per-kilometre rate. It tells you, before you get in, whether traffic will be expensive or merely slow.
Watch when the number moves. In free-flowing traffic it should step in rhythm with distance covered. Stopped at a light it should step in rhythm with the clock. If the number is climbing at a steady rate that ignores both, that is worth noticing.
If you want to see the mechanism running live rather than take this article's
word for it, Meterless measures a ride from your own phone's GPS
and shows which rate is currently accruing — on distance or on time — as the
vehicle crosses the changeover speed. It is an estimate for the passenger, and
deliberately never a meter: taximeters are legally controlled instruments, and
this one is not certified, sealed, or claiming to be.