The number of drivers found by police to have taken illegal drugs has soared in recent months.
But campaigners believe that current prosecution rates may only be scratching the surface of a much more widespread problem.
In March, the coalition government introduced a new offence of drug driving in England and Wales.
This gives the police powers to use new “drugalyser” equipment on any motorists suspected of having taken illegal drugs, whether or not their driving appears to be impaired.
Prior to March, officers were obliged to show that the drug-taker was unfit to drive before they could prosecute.
But now, convictions can be secured whenever suspects have sufficiently high concentrations of illegal substances in their blood.
As a result, police forces around the country have reported drug-driving offence rates significantly higher than in previous years.
In Cheshire, for example, prosecutions are up by more than 600%, while in London they have doubled.
Drug drivers can be fined up to £5,000, banned from the roads for a year or more, or even put in prison.
But a the RAC's Simon Williams said the true extent of this crime could be much greater.
“Despite the apparent surge in drug-driving offences being recorded as a result of the ‘drugalyser’ being introduced, we suspect conviction rates may actually understate how widespread drug-driving is.
“Those suspected of drug-driving are also driving under the influence of alcohol and it is easier and cheaper for the police to test for drink-driving.
“Penalties for the two offences are the same and, in cases where motorists are suspected of being affected by both drugs and alcohol, police will tend to prosecute only for the latter.”
At the moment, police forces in England and Wales are mounting their annual crackdowns on drink-driving during the festive period – but this year, potential drug-driving is also under scrutiny.
Detective Chief Superintendent Paul Rickett of the Metropolitan Police Service in London said:
“The Met is taking positive action during this campaign and at all times, to catch those who break the law in this way and bring offenders to justice.
“Driving a vehicle when over the prescribed limit of drink and/or drugs, legal or illegal and is an offence under the Road Traffic Act.
“Not only are you breaking the law but you are posing an unacceptable risk to yourself, your passengers and other road users.”
The new laws also put limits on the levels of certain legal prescription drugs that drivers can have in their systems, including certain strong painkillers.
Limits on legal drugs
Williams at the RAC added: “Of the motorists surveyed for the RAC Report on Motoring 2015, 6% said they had driven under the influence of drugs, whereas in 2014 the rate was just 2%.
“This may be because the introduction of the new drug-driving legislation has increased awareness of the effects of the 12 prescription drugs specifically identified.”
But the government says that most people will be able to drive safely provided they stick to the level of medication prescribed by their doctors.
Robert Goodwill, road safety minister, said: "If you are taking your medicine as directed and your driving is not impaired, then you are not breaking the law and there is no need to worry.
"We advise anyone who is unsure about the effects of their medication or how the new legislation may affect them to seek the advice of their doctor or pharmacist.”