Avant, hand-operated cameras can work for vehicles travelling either towards or away from them. They work by laser, so in the event of a car travelling away the speed is recorded as a negative, or receding value. Still perfectly valid though
They are usually used in the 'toward' direction because the driver can usually be partially identified - which can either aid a keeper in identifying the driver or identify someone who is taking points for the real driver. This is not required though - it is usually enough that the car was speeding.
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