The amount of time it takes an average driver to respond to a hazard can be almost twice that assumed in the new Rules of the Road, according to an unpublished report.
A study on braking distances for the Road Safety Authority (RSA) found that despite improvements in the braking capacity of cars during the 12 years since the old rule book, driver reaction times have increased, largely due to more distractions.
An RSA spokesman said the Transport Research Laboratory (TRL) in England had been commissioned to study how long it takes a car traveling at a certain speed to come to a stop.
In 1995 the stopping distances associated with different speeds assumed a reaction time of 0.66 seconds.
However, when the TRL researchers examined whether this reaction time was still valid, they found it only applied to drivers who are highly alert and aware of the need to react.
For all other drivers, the reaction time should be assumed as being between 1 to 1.5 seconds.
To explain the significance of this difference the TRL report says a driver travelling at 30km/h is expected to be able to bring a car to a stop within 10.8 metres on a dry road, based on the 0.66 second reaction time.
However, if reaction time is increased to 1 second, the stopping distance increases to 13.3 metres, or by almost two car lengths. At 100km/h in wet conditions the difference in stopping distances increases from 122 metres to 132 metres.
Mr Farrell said drivers were increasingly distracted in their cars due to music, passengers, hands-free phones and roadside advertising. This was resulting in slower reaction times he said.
The TRL report also notes that meeting the stopping distances in the Rules of the Road - which are the same as those recommended in Britain - requires a severity of braking that most drivers would be uncomfortable with.
Mr Farrell agreed that, the stopping distances published in the Rules of the Road do assume emergency braking, although this is not specifically stated in the document.
Asked why the Rules of the Road used the shorter, emergency reaction time, Mr Farrell said the published braking distances "give motorists an idea of how long it will take them to actually stop under emergency conditions".
A motorist's reaction to a hazard has two elements: their perception of the hazard and how quickly they respond, or brake. These two factors are those which can suffer the greatest impairment due to alcohol, fatigue, drugs or poor weather conditions.
