Brynglas tunnels 'cost £31m in lost time' a year
The M4's Brynglas tunnels at Newport are the most-congested stretch of inner city motorway in the UK away from the M25, new traffic data has revealed.
Traffic data firm Inrix said junction 25 to junction 26 on the M4 is the UK's fourth most-congested stretch and in the Europe's top 50.
Jams average 57 minutes and four miles (6.4km) long, costing drivers £31m a year in lost time, it says.
The Welsh Government wants to build a £1bn M4 relief road to ease congestion.
A public inquiry into building a six-lane motorway to the south of Newport has heard from a government barrister who said the current M4 at Newport was "not fit for purpose".
The Welsh Government's appointed traffic consultant told the inquiry the tunnels were "at or reaching capacity".
Dr Graham Cookson, chief economist for Inrix, said: "Our data shows Brynglas tunnels to be one of the worst traffic hotspots in the UK, with congestion here costing drivers £31m in lost time annually.
"Congestion costs the Welsh economy billions, hampers business efficiency, and wastes drivers' time and money."
"Despite these issues, congestion is in fact a good problem to have as it's a strong indicator of prosperity and economic growth.
"However, in order to resolve these issues we must find a solution before the negative effects of traffic detract from the positive causes."
The tunnels opened 50 years ago on Friday and were originally built for just 30,000 vehicles a day.
Currently, 78,919 vehicles a day on average use that stretch of road and there have been 1,477 traffic jams around the tunnels in the past 12 months, about four jams a day on average.
Inrix said the longest distance traffic jam at the tunnels in the last year was over 14 miles on the eastbound stretch in October where tailbacks reached Magor in Monmouthshire.
Statistics also showed the longest duration of a queue was eight hours as an eight mile congestion clogged the tunnels and the M4 corridor in October.