The busy section of the M4 is closed for several hours each night to carry out improvements and we were able to go into the tunnels on foot via Wales http://ift.tt/2dPk581

https://showcavesuk.wordpress.com/2016/10/19/this-is-what-it-looks-like-inside-the-closed-brynglas-tunnels-as-work-is-carried-out/