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The Brynglas Tunnels (sometimes known as the Crindau Tunnels) are situated on the M4 motorway to the north of Newport in Monmouthshire, South Wales, between junctions 26 and 25A. They are twin-bored tunnels, each carrying two traffic lanes, opened in May 1967 as part of the M4 approach roads to the Severn Bridge. The tunnels are 370m long. It is reported that several houses above the tunnel were demolished as a direct result of ground movement caused by the construction of the tunnels.
Road users are warned on the approaches to use dipped headlights in the tunnels. Both tunnels have overhead lane control signals, though they are only used during a contraflow. The A4042 Newport - Abergavenny road passes over the eastern openings of the tunnels on the route of the old Monmouthshire western valleys railway line.
The tunnels are now the only stretch of D2 on the M4 between junction 3 in West London and junction 38 south of Port Talbot. Originally, the rest of the M4 in the area was also D2, but was widened to D3 in the 1980s. Consequently, the tunnels are now a significant bottleneck, as widening is prohibitively expensive. The preferred solution is to provide an alternative bypass of Newport to the south (see M4 Toll), but this has never progressed as far as a definitive construction date.
In July 2011, a severe lorry fire in the westbound tunnel resulted in closure of the tunnel for several days whilst temporary repairs were carried out. This included temporary strip lighting which remained in place until 2014, until a major refurbishment could be carried out. This meant that for three years, the eastbound tunnel was lit by orange lighting, the westbound by white.