Meaning speed chart
The graph in your dashboard shows the speed of the cars. To begin with, it's important to know that speed statistics are indicative. Measurements may differ by 10% from effective speeds.
The figure shows, in a bar chart, the percentage of the cars per speed level, for the period you have selected at the top of the page.
The way Telraam measures speed
The speed measurements are distance independent, because we can't know for every unique Telraam how far it is from the road. 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 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. If you want to know more about this topic, please read the article "Speed measurement and unrealistic speed data ".
Accuracy of these speed data
These figures are not compared to a calibrated speedometer. Theoretically, we expect there to be a limited error for properly functioning Telraam devices, if you look at them for a sufficiently large time interval. The speeds are primarily indicative and provide a good basis for comparison with other streets.
Extremely high or low speed measurements
It is possible you see a percentage of cars in the graph with a speed higher than 70km/h or lower than 10km/h for example, while you feel like this can't be possible. These are probably so-called outliers; measurements in which something has gone wrong. We don't filter these errors.
If the statistics indicate that a number of cars are travelling at less than 10 km/h, this is probably due to a misclassification. For example, a group of cyclists that is classified as a car. If you would like to know more about this classification, you can read more about it here: "How are the different types of vehicles classified and differentiated? ". The above linked article "Speed measurement and unrealistic speed data" tells more in detail about the reasons why the speed measurement data could be unrealistic.
If you suspect that there is a problem with the speed measurements, report it to the Telraam team by sending us a mail at firstname.lastname@example.org
Is my Telraam a speed camera after all?
No, certainly not. Your Telraam cannot make a statement about the (calibrated) speed of individual cars. The measurement error can be large at the level of individual vehicles, but that error can be in both directions (faster or slower than in reality). This error corrects itself at the level of a group of cars, as there are more measurements. Telraam can therefore give you an idea about the speed regime of a large number of vehicles.
What about large vehicles?
Telraam cannot measure the speed for large vehicles because the measurement assumes that objects within one group have the same typical length. This is relatively true for cars (the length of any given car will always be very close to the "typical car length"), but larger vehicles range from larger SUVs to long trucks and buses, which is a very broad scale for length.