Stopping distances in the Highway Code are woefully short because they underestimate drivers' thinking time, road safety campaigners claimed.
A study commissioned by charity Brake estimated that it takes an average of 1.5 seconds to spot a hazard and apply the brakes, more than double the figure of 0.67 seconds used in the Department for Transport's book.
The new study calculated that the stopping distance of a car travelling at 40 miles per hour is 51 metres, compared with the figure of 36 metres in the Highway Code.
This is the equivalent of an extra 3.75 car lengths and Brake urged the Government to increase stopping distances in the book "as a matter of urgency".
The charity's spokesman, Jason Wakeford, said: "These figures suggest stopping distances taught to new drivers in the Highway Code fall woefully short.
"A true understanding of how long it takes to stop a car in an emergency is one of the most important lessons for new drivers.
"Understanding true average thinking time reminds all drivers how far their car will travel before they begin to brake - as well as highlighting how any distraction in the car which extends this time, like using a mobile phone, could prove fatal."
RAC spokesman Rod Dennis described the new figures as striking and called for them to be taken seriously.
He said: "From time to time, new evidence will come to light that means it is necessary to update the Highway Code and perhaps this is one such instance.
"While the ability for cars to be able to brake more quickly has improved, our reaction times clearly haven't."
:: Here are the average stopping distances of cars according to a study commissioned by Brake and conducted by the Transport Research Laboratory:
Speed - Brake/TRL study - Highway Code - Difference
20 mph - 19 metres - 12 metres - Seven metres
30 mph - 34 metres - 23 metres - 11 metres
40 mph - 51 metres - 36 metres - 15 metres
50 mph - 71 metres - 53 metres - 18 metres
60 mph - 95 metres - 73 metres - 22 metres
70 mph - 121 metres - 96 metres - 25 metres
The Department for Transport said in a statement that it would "carefully consider these findings".
It added: "We have some of the safest roads in the world and we are always looking at ways to make them safer.
"Stopping distances in the Highway Code are intended to inform motorists of the thinking and braking time needed to stop a vehicle at different speeds."