Just doing some calcs, I assume you would need 2 seconds minimum reaction time from an alert to drop say 10-15kph
at 70kph = 38metres
at 100kph = 52 metres
at 120kph = 66 metres

So Poliscan measures anything between 15-75 metres (40m from what jezzamcbezza was told) so you would need an alert minimum of

at 70kph = 78metres - 113metres
at 100kph = 92 metres - 127metres
at 120kph = 106 metres - 141 metres