Could some recommend a affordable speed detector with good speed accuracy?

This will be used in subway tunnel, measure the subway car travelling
speed, need to be pretty accurate. The speed detector can only be
mounted on one side of the track, so the cross tunnel laser is not
allowed.

Method of measurement is not limited, but need to be safe one that
will be allowed in install in the subway tunnel. The subway car mostly
will travel at 30~80 km/hour, we need to know the speed as accurate as
possible.

-- Youda
--=20
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