An eight-week motorway contraflow held 99.97% uptime, with no signal-related incidents.

The brief.

National Highways needed an eight-week contraflow on the M25 J7 anticlockwise to allow full-depth resurfacing of the inside two lanes. The default plan called for fixed 200-mm signals on temporary gantries, with portable kit as backup.

The contractor asked whether four portable units could replace the gantries entirely, on the grounds that the unit cost was lower and the audit trail was richer. National Highways’ incident review team had reservations — the category had a reputation for failing on long-duration motorway work.

Ardent agreed to a stepped commissioning: two units for the first week, four for the remainder, with the contractor’s choice to revert at any point.

Four units, paired, with hot-spare battery rotation.

Two units at the contraflow taper, two at the merge, all on 120-second cycles with vehicle actuation disabled — on the M25 the queue is the steady state, not the exception. Battery packs rotated on a 72-hour cadence; depot van picked up depleted packs nightly.

Telemetry was ported into National Highways’ incident dashboard via the read-only API. The duty incident officer could see all four units at a glance, with cycle state, battery level, and tilt history. The unit nearest the merge was paired with a hot spare in the central reservation, on standby for swap.

The hot spare was not used. It was visible in the audit trail, which mattered more than the swap itself.