Birmingham's main city centre tunnels have closed for six weeks to allow the first major work in their 40-year history to be carried out.
The St Chad's and Queensway tunnels, on the A38 between St Chad's Cathedral and The Mailbox, shut at 22:00 BST.
Steve Brittan, Birmingham Chamber of Commerce's president, has warned it will cause "chaos" in the city centre.
The tunnels, which are due to reopen at 06:00 on 2 September, will also shut for six weeks for more work next year.
Over the next six weeks, nearly 1,000 new lights will be installed in the tunnels and about 21,000 sq m of fire protection will be added to the walls and ceiling of the tunnels.
Engineers said they will also improve fire escapes and the general appearance of the tunnels.
Amey, which is carrying out the work on behalf of Birmingham City Council, said it looked at various options of how to carry out the work and shutting the tunnels completely for six weeks was the cheapest.
It said traffic levels were usually up to 20% lighter during the school summer holidays, which start in Birmingham on Wednesday.
Mr Brittan said having the two tunnels closed for the full six weeks would make traffic "horrendous".
He said he would boycott the city centre while the work was being carried out and urged other drivers to do the same to alleviate congestion problems.
Broad Street closure
The tunnels have already been shut overnight for four weeks and will close during the night again for two weeks after the complete closure.
Edgbaston councillor Deirdre Alden said she had found out via a question in a committee meeting - and not through the information released by the council and Amey - that part of Broad Street would also be shut while the work was carried out.
Ms Alden, a Conservative councillor, said the inbound part of the road between Paradise Circus and the Hyatt Hotel would be shut to cars over the six weeks although buses and taxis would still be allowed through.
"It's going to be mayhem in parts of the city centre," she said.
"I can understand them not wanting Paradise Circus to take any extra traffic, but this is going to make things worse."
Along with road diversions, extra measures have been put in place by Amey, National Express and Centro to encourage drivers not to come into the city centre.
A 500-space park-and-ride site has been set up at Birmingham City University's Perry Barr campus, extra buses are being put on and three "bike trains" are being set up to allow people to cycle in large groups into the city.
The refurbishment of the tunnels is being paid for as part of the city council's 25-year £2.7bn contract with Amey to maintain Birmingham's roads, footpaths, bridges, tunnels, street lighting and traffic control systems.
Amey has not said exactly what proportion of that is being spent on the tunnels.
Next year, work will concentrate on electrics within the tunnels.
The council and Amey have set up a website giving details of the tunnel closures.