Document: Can Watch Your Speed Signs be used to count vehicles?

[radumas]

This is a question that comes up.

The below is a blend of our analysis, personal observations, and responses from the vendor that should be added to the documentation:

The signs use a Doppler radar technology which means they can detect only speed and direction. The sensor cannot see the distance or azimuth to the object and therefore cannot count how many moving objects are if they are within +-5kph speed. Based on personal observations they detect smaller road users such as cyclists and scooter users. The signs can make only an approximation about traffic volume and classify it by speed and cannot count precisely on a high-volume road. If the sensor detects a moving object during some speed-dependent interval of time, it adds it to the statistics as one event (even if it was a bunch of cars in the chain).

From an analysis we did in 2021 comparing a few signs to ad hoc counts the general conclusion is that they tend to undercount on higher volume, higher speed multi-lane streets and overcount on lower volume single-lane streets. We have used changes in counts over time for a panel of locations as a volume index to monitor trends during the stages of the pandemic with the caveat that they are installed in school safety zones which tend to be in in residential neighbourhoods and not on arterial streets.