Motorways are not safe enough to cope with raising the speed limit from 70mph to 80mph, according to a report.
The government intends to consult on introducing an 80mph limit on English and Welsh motorways.
But England's motorways do not provide enough protection to drivers and car occupants for an increase in the limit to be considered, the report from the Road Safety Foundation (RSF) said.
It added that there would be a rapidly-rising risk of shunt crashes from the sheer volume of traffic using the motorways.
The report also said there were widespread faults in the protection given to drivers to prevent them running off the road on motorways.
The RSF also cited the problem of pedestrians on the hard shoulder, roadworks, extreme weather and spillages from vehicles.
In addition, there would be negative economic effects from a higher speed limit, including increased vehicle operating costs through higher fuel consumption; increased crashes and crash severity, resulting in raised crash costs; and the increased cost caused by delays from crashes.
The foundation said it recognised the argument that respect for the 70mph limit was poor and it did not dismiss proposals to raise the limit.
But the report concluded: "Drivers who want to are already travelling at 80mph when they can. Economic benefits only arise if '80 means 90' and opinion surveys show no public support for that.
"However, large economic benefits arise from fixing the motorways systematically rather than raising the speed limit."
RSF director Joanne Marden said: "Our cars provide 4-star or 5-star crash protection but too many of our motorways rate only 3-star, with major weakness in run-off protection."
Proposals to bring in 80mph limits were first put forward in autumn 2011 by the then Transport Secretary Philip Hammond. If a higher limit is introduced, there would still be some sections of motorway where the limit would stay at 70mph.
Two-thirds of AA members support an increase in the limit to 80mph.
AA president Edmund King said: "Driving at 80mph in a modern car in good weather at a safe distance from the car in front on a well-engineered motorway is perfectly safe.
"Driving at 50mph tailgating the car in front is not. Driving at 75mph on a sub-standard stretch of motorway without a decent central reservation or run-off areas may not be safe."
He added that 80mph was "already accepted by most as the de facto limit so it is better to legalise this limit on the safest stretches of motorway and enforce it".
Roads minister Mike Penning said: "The Department for Transport is carrying out work to assess the potential economic, safety and environmental impacts of trialling 80mph speed limits on motorways where variable limits are currently in place.
"This work is on-going and no final decisions have yet been taken about which stretches of motorway would be included in any proposed trial.
"We plan to bring forward detailed proposals and start consultation during the next few months."