How does the vehicle reachability work?

By default every vehicle is unreachable to begin with. Once the vehicle receives a telemetry the vehicle is set to 'Reachable' and gets a 24 hour countdown.

If there has been no telemetry data for 24 hours the status changes back to 'Unreachable'.

Why would we not receive telemetry data for 24 hours?

It could be that the vehicle has been parked somewhere where there is no signal (underground for example). It could also be that the device has been unplugged but we will not receive an alert for that unless it has been plugged back in (this raises a 'device unplugged' alert).

How do I check the reachability?

To check the reachability of the vehicle go to Operations --> vehicles --> click on the vehicle registration. You can see the status on the header page under the status 'connectivity'.