Drivers who drive it, and journalists who report on it, will be very familiar with the number of accidents and traffic jams on the M4.
It can seem in some weeks that there is an incident every day or two that causes long delays and, in the worst cases, a serious or fatal injury. Last month, there were four accidents on the same stretch in less than a week.
There were no serious injuries but crashes have caused hours of delays for commuters and the problem seems to be getting worse. For years, the pinch-point around Newport , in particular the Brynglas Tunnels, have constricted traffic flow and sent blood pressures soaring.
In 2018, there were two fatal accidents, five serious and 10 other accidents recorded between junction 29 at Cardiff and junction 23 at Magor.
It is the busiest part of the motorway in south Wales. Department of Transport figures show that junction 27 at High Cross sees nearly 122,000 vehicles a day.
The sheer volume of traffic could be a factor in the number of accidents, but some commuters are blaming the variable speed limits introduced to that section of the motorway in 2016, while experts say the motorway was never built for the number of vehicles that actually use it. Police say poor weather conditions are also a contributing factor.
— Read more on www.walesonline.co.uk/news/wales-news/m4-wales-reasons-congestion-number-17086504