Northampton Safety Camera Partnership use Truvelo front-facing piezo
triggered cameras. I've visited the manufacturer in South Africa and,
to the best of my knowledge they DO flash if you're dumb enough to
speed as you approach one ! The flash is magenta to avoid blinding
approaching drivers. This was developed in order to get the product
through Home Office Type Approval (HOTA) and agreed by the Police
Scientific Development Branch who are the test body for such approvals.

You _do_ see a flash with Truvelo, you do _not_ see a flash with SPECS
as the video cameras are constantly live and use IR filtered lamps and
lenses because that is the best way of delivering 24/7 images to the
underlying Automatic Number Plate Reader (ANPR) systems - assuming the
numberplates being viewed are retroflective - which they ARE in the UK.
As Truvelo cameras do NOT use ANPR and are designed to provide the best
image of an approaching, offending vehicle, there would be no sense in
using IR.