There is no evidence to suggest how quickly — or slowly — a woman charged with running over twin brothers reacted, according to a collision expert.Randolph and Rudolph Smith, 36, were both dragged more than 40ft along Woodlands Road following the early morning collision on January 29, 2012.Tracey Pitt, 51, has denied six charges in connection to the crash, including two counts of causing grievous bodily harm by driving while impaired.Supreme Court previously heard that Ms Pitt, who was arrested at the scene, admitted having two glasses of wine but refused to submit to a breath test.Both brothers have told the court they do not recall the crash itself, only waking up in hospital, but witnesses testified that they saw the brothers wrestling on the ground in the southbound lane moments before the crash.On Thursday, traffic collision investigator Emerson Carrington told the court that based on the injuries sustained and damage to the car, the twins were sitting, squatting or lying in the road when they were struck. They were then dragged more than 40ft, caught in the car’s undercarriage.Returning to the stand yesterday, Mr Carrington said he was unable to determine the speed at which Ms Pitt was travelling or her perception response time — the time taken to recognise a hazard and react.He explained that the average perception response time is 1.7 seconds, but in the case of someone who is impaired the time would be longer.During cross examination, defence lawyer Elizabeth Christopher put to Mr Carrington that, based on the drag marks at the scene, Ms Pitt’s response time would be measured at 1.2 seconds.However Mr Carrington said the estimate was “totally flawed” because it used drag marks found at the scene as a substitute for brake marks, which were not found at the scene of the crash.He also rejected the idea that the evidence suggested Ms Pitt reacted “promptly” to stop her car, reiterating that there was no evidence of her reaction time.“We would have to assume a lot of stuff,” he said. “In my opinion there are too many unknowns. We don’t know the speed of either of the vehicles, we don’t know the time the vehicles passed each other, and we don’t know the driver took 1.7 seconds to react.”Ms Christopher suggested to Mr Carrington the twins were difficult to see due to their position in the road, the colouration of their clothing and the glare of lights from a passing vehicle.The investigator responded: “I don’t know the colour of clothing they were wearing, and from my recollection the collision scene was well lit from street lamps. Additionally headlights on the motor car were on.”And he said that while glare from a passing motor car could effect reaction time, he could not say if it would cause faster or slower reactions.