North Yorkshire County Council's transport, economy and environment scrutiny committee also heard the number of slight injury accidents rose by some 60 per cent over 2019.
Fiona Ansell, the road safety team leader for the council, told councillors the rise in serious casualties to 411 from 298 was the first increase in the figure the county had seen in decades.
The meeting was told the council was analysing collision and traffic flow data as North Yorkshire Police felt it was related to less traffic on the roads, leading people to travel faster, so the severity of accidents had been greater.
During the first lockdown last April, traffic volumes on North Yorkshire's roads fell to less than 40 per cent of those the previous year, but by August the number of vehicles on the roads had returned to the previous year's average.
The police launched a crackdown on speeding during the first lockdown in spring after concerned residents reported hugely excessive speeds in their communities.
Police deployed in force to key locations in communities, where they dealt with motorists doing twice the 30pmh speed limit, and on busy A-roads, where several drivers were dealt with for doing speeds of more than 130mph.
In November, police said speeding in communities across North Yorkshire had increased again during the second lockdown, with some motorists clocked at almost twice the speed limit.
After the meeting, Coun Stanley Lumley, the committee's chairman said while the results of the authority's investigation would come too late to guide road safety advice for the latest lockdown, it was clear motorists needed to take more care and be more vigilant.
He said: "It might be the case that as people haven't been driving for some time their reaction times and skills are lacking, and it's even more relevant for people on motorcycles and bikes due to the likelihood that you will be more seriously hurt."
"Clearly these figures are of great concern. These serious accidents have to be looked at and questions asked to learn the reasons why. It's a trend we certainly don't want to continue.
"We are in extremely unusual times and figures will be skewed for many reasons relating to that, one of which might be the roads are so much quieter so people are driving faster and taking more risks.
We also get a tremendous influx of people coming to the county to enjoy our wonderful scenery and drive our roads. As soon as lockdown restrictions were eased we were overwhelmed in Nidderdale as people were coming out of the cities where they had been trapped in conditions far worse than we have rurally."