How does a taxi meter work? Parts, calculation and checks

A taxi meter works by measuring two quantities, the distance the car covers and the time the journey takes, and converting them into money according to the official fare stored in its memory. It receives the distance as electrical pulses generated by the vehicle itself as it moves; the time is counted by an internal clock. While the taxi is moving freely, it charges by distance; when it is going slowly or has stopped, it charges by time. All of this happens in a type-approved, sealed device that undergoes regular checks so that nobody can tamper with it.

Behind that display adding up the cents there is more engineering and more control than you might think. Let us take it piece by piece.

Although today it is usually a compact unit built into the dashboard or the rear-view mirror, a modern taxi meter has several elements that work together:

Distance, in pulses

The vehicle generates an electrical signal as it moves. Each car produces a different number of pulses per kilometre, depending on its transmission and the size of its wheels, so when the meter is installed it is calibrated with what is known as the vehicle constant: how many pulses equal one kilometre in that particular car. From then on, the device only has to count pulses to know how many metres it has travelled.

Time, with a clock

In parallel, the internal clock keeps count of the seconds. Time has two uses: charging for slow or stationary stretches and knowing which fare applies according to the time and day, because in Madrid the fare for nights, weekends and bank holidays is different from the weekday daytime fare.

This is the concept that best explains how a taxi meter behaves and the one that raises the most questions. The device does not add up distance and time at once: at any given moment it charges for whichever of the two is higher.

The boundary between the two modes is called the crossover speed and is obtained by dividing the hourly rate by the per-kilometre rate of the fare. Above that speed, covering a kilometre costs more than the time it takes, and the meter charges by distance. Below it, time weighs more, and it charges by time. That is why:

• On a clear avenue, the amount goes up in step with the kilometres.
• In a traffic jam or at a red light, it keeps going up even though the car is not moving, in step with the hourly rate.
• A short journey at rush hour can cost more than a longer one in the early hours.

We set out the full calculation, with the initial charge, kilometres, time and supplements, in how taxi fares are calculated in Madrid.

Over the course of a journey, the device goes through several positions. Knowing how to recognise them helps you check that everything is in order:

The meter must be started when the journey begins, not before, and switched to “to pay” on arrival at the destination. On journeys with a fixed fare, such as the airport run from inside the M-30, the amount no longer depends on the counter: what you pay is the amount set for that journey.

A taxi meter is a measuring instrument subject to metrological control, just like a petrol pump or a shop scale. That brings several safeguards:

1. Type approval. Only models approved under European regulations on measuring instruments can be installed.
2. Seals. The device and its connection to the vehicle are sealed, so any tampering leaves a trace.
3. Regular checks. An authorised body verifies that it measures correctly, and also after a repair or any change that affects the measurement.
4. Fare updates. Every time the City Council approves new fares, the meters are reprogrammed to apply them.

That is why, in an official taxi, there is little point in distrusting the device; what you should watch is that the service really is a licensed taxi. We explain how to identify them in the risks of a fraudulent taxi.

• Look at the active fare when you get in: it should match the day and time.
• Check that it starts at the initial charge and not with an amount already built up.
• Check the supplements on the display when you arrive: they must appear in the fare table displayed in the car.
• Ask for the printed receipt if you need it or want to complain. What you can ask of the driver is set out in taxi passengers’ rights.

If you are curious to know where this device comes from, from the odometers of antiquity to the first mechanical taxi meter, we tell the story in the origin of the taxi meter.

When you do not need to watch the meter

On pre-booked airport transfers we work with a fixed price by area, without the meter: from inside the M-30 it is €33, and the amount for each neighbourhood and town is on our airport fares page. The same goes for the return: with a pickup at the terminal, you know what you will pay before you land.