# How does Telraam measure speed?

### Telraam measures the time it takes a car to travel its own length - which does not depend on the distance from which the car is perceived - and assumes the car has a standard length, i.e. the length of a typical car

Telraam cannot measure speed directly like a typical police speed gun (a.k.a. a small Doppler radar), so we need to use an indirect method to derive velocities.

In an ideal world, if we knew the distance between the camera and the passing objects, or the length of the road covered by the field of view of each individual Telraam unit, then we could calculate speeds from measured travel-times. For example speed could be easily calculated from the time it takes a car to travel along a known length on the road.

Unfortunately each Telraam unit’s location is unique, so the distances between each Telraam unit and their host windows are different, and it would not be practicable to ask each user to actually measure this distance between their unit and the middle of the street. Furthermore, the closer a multi-lane street is to the Telraam unit, the bigger the difference will be between the distances that are present between the Telraam and the individual lanes.

As building fronts in urban areas are often only separated by a narrow sidewalk from the road surface, such differences can easily reach up to 50%, meaning that a single distance value (between the Telraam units and, for example, the middle of the road) would cause significant errors when calculating speeds of objects that pass through the individual lanes.

To make our measurements distance-independent, we came up with the following clever solution:

Telraam measures the time it takes a car to travel its own length

This does not depend on the distance from which the car is perceived - and assuming the car has the length of a typical car (a value we calculated from the most popular cars sold in recent years in Belgium), we can now calculate its speed. Of course this method has its drawbacks too.