The A43 Corby relief road has been open for 12 months and has seen daily traffic levels rise from 8,000 to 11,300 since it opened last May.
But many lorries and cars are still taking the old road through Geddington.
Michael Clarke, from the county council highways department, said: "Old habits die hard and some sat-navs have still not been updated for the new route".
He is pleased more vehicles are now choosing to use the new road, which was expected to carry 25,000 vehicles a day.
Poor signage has also been blamed for the lack of cars.
"It's developing just like the M1 when it opened. It had little traffic at first but is now well used," Mr Clarke said.
"When the penny drops and drivers use the road they realise how quick it is."
He said the road formed part of plans for new roads which bypass the villages of Flore and Weedon near Daventry.
Mark Rowley, chair of Geddington Parish Council, said the road is making some difference to congestion in the village.
He wants the county council to do more to persuade local transport companies to use the new relief road.
Andrew Howard, managing director of a logistics firm at King's Cliffe, said his drivers were encouraged to use the road from the day it opened.
"It is faster and less stressful for our drivers. We made them aware of its advantages straight away," he said.
"It's a faster route to customers in the north of England."

There were also golds for James Crisp, who won his fifth European title in the S9 100m backstroke, while Ollie Hynd led home a GB one-two in the S8 400m freestyle with Josef Craig taking silver.
Hannah Russell (S12 50m freestyle) and Stephanie Millward (S9 100m backstroke) added silvers to the GB medal haul.
Rodgers beat her previous best of 36.01 seconds, set in 2012, with an impressive 35.90 to triumph by 2.82secs and take her third consecutive title.
The 32-year-old, who missed out on a medal in the event at London 2012, is benefiting from a new butterfly technique where she uses just one arm and although most of her main rivals are from outside Europe, she was pleased with her time.
"I've been working quite hard on that one," she said. "Last week was a bit of a mixed bag for me at trials so it's really nice to come out and get a good time.
"It's great to see the hard work paying off. If PBs and records happen all the time then you take it for granted but it's such an amazing feeling."
Sheffield's Crisp, 33, who won his first European title back in 1999, used all his experience over the second 50m to finish 0.8secs ahead of Poland's Patryk Biskup with team-mate Lewis White finishing fourth at his first major championships.
"I kind of see the last 19 years as a bit of a warm-up to this year," he said. "This is the year that counts and it's good to know I am there or thereabouts and fighting for the medals still.
"I have to change my training a little bit now I am in my thirties but I feel strong and fit."
S8 400m silver medallist Craig was competing just a day after being disqualified from the S8 100m freestyle final because his tattoo breached advertising regulations.
He returned to the pool on Monday with the design covered up.
Out of date sat-navs have been blamed for motorists' slow uptake of a new £35m road in Northamptonshire.
[NEXT_CONCEPT]
Susie Rodgers improved her own S7 50m butterfly European record to win one of three golds for Great Britain on day two of the IPC Swimming Open European Championships in Madeira.