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The Brynglas Tunnels were opened to traffic in May 1967. They convey the M4 motorway for 370 metres (about 1200 feet) under Brynglas Hill. The twin-bored tunnels were the first tunnels on the UK motorway network. The motorway either side of the tunnels has been widened to 3 lanes each way, but the motorway through the tunnels remains as built, 2 lanes each way. The tunnels were not widened because of the risks of causing structural damage to Brynglas houses above the tunnels. Several Brynglas houses had to be demolished because of such damage caused when the tunnels were built.
Thus the tunnels remain a bottleneck on the M4 through Newport. On the occasions when there is an accident in a tunnel, the necessary temporary closure of the M4 in this area causes severe traffic congestion in neighbouring parts of Newport.