It came after an accident at 1.13am today (Wednesday) when a silver Mercedes car travelling eastbound on the A45 entered the sliproad of the junction with the A43 Lumbertubs Way.
A police spokeswoman said: "As the vehiclr approached the traffic light-controlled roundabout, it left the carriageway to the left hand side, striking roadside furniture and trees before coming to a rest down an embankment."
The male driver was taken to University Hospitals Coventry with "serious, life-threatening head injuries".
Witnesses or anyone with information is asked to call the Drivewatch Hotline on 0800 174615.
The county council's Highway's team estimated the area near the A45 westbound exit to the A43 (Riverside Retail Park roundabout) will be congested until about midday.
Due to the resulting congestion Stagecoach said there were delays to its X46, X47, 7, X6, X7 and X44 services.