A £4M project to unblock one of Oxfordshire’s worst traffic blackspots will finish this week, ending months of misery for motorists.

Traffic engineers believe improvements to the M40/A34 junction at Wendlebury near Bicester will combat severe southbound tailbacks on the motorway.

And they hope it will stop drivers rat-running through nearby villages to avoid the jams.

But a second phase, to improve the junction for northbound traffic, appears to have stalled through lack of funds.

Work to widen southbound exits began in August and is set to finish on Friday.

The Highways Agency, which carried out the project, has warned it will take a few weeks for drivers to see the benefits.

Drivers have put up with narrow lanes, 40mph speed restrictions and frequent overnight closures during work to allow the southbound motorway exit and the turn from the roundabout on to the A34 to be widened to three lanes. Jams on the southbound M40 have at times stretched back almost to junction ten at Ardley.

Highways Agency spokesman James Wright said: “It has gone well and we are looking at reopening it at the end of the week. It will be a little time before we can assess the benefit as it takes traffic time to get used to it.

“In the short term, the first rush hour could be a little worse as it takes time for traffic to settle down.”

The planned £6.5m second phase – to improve the northbound approach to the junction on the A34 and to widen the A41 between the roundabout and Bicester – remains on hold because of funding concerns.

Mr Wright said phase one had been designed to work in isolation and tackle the main cause of congestion, southbound M40 traffic heading for the southbound A34.

He said alleviating that pressure would improve journey times for motorists travelling north as well.

He added: “It is that (southbound) movement that is the cause of tailbacks and causes the flow of the whole roundabout to slow down.”

Kathy Sharpe, the vice-chairman of Wendlebury Parish Council, welcomed the news, but said members would continue to press for phase two to go ahead.

She said: “We would hope to see an improvement in the rat-running.

“Phase two is meant to improve the flow of Bicester Village traffic at peak times.

“It would be a shame if that’s not going ahead.”