Police speed guns accurate: expert

PUBLISHED : Friday, 18 January, 2008, 12:00am
UPDATED : Friday, 18 January, 2008, 12:00am

There can't be an error of 20km/h to 30km/h with lasers used by HK force, says professor

Laser guns used to measure vehicle speed are accurate to within 7km/h, says a professor of physics who advises the Hong Kong police on their use and has been testing them for the force for nine years.

'I have full confidence in the guns' accuracy,' Tam Wing-yim said.

Professor Tam was speaking a week after Lai Sun Development chairman Peter Lam Kin-ngok succeeded in having a speeding charge reduced when the officer who clocked him allegedly doing 114km/h in a Ferrari admitted in court he had breached police procedures for using laser guns.

The charge against Mr Lam was amended to say he was driving at 79km/h in a 50km/h zone. The amendment meant Mr Lam no longer risked being suspended from driving, and he pleaded guilty and was fined HK$450.

Michael Clark, a British expert who testified for Mr Lam, said after the case that it was clear that the machine had not been set up properly.

Stressing that he did not want to comment on individual cases, Professor Tam, of Hong Kong University of Science and Technology, said: 'There is no way there will be an error of 20km/h or even 30km/h.'

Professor Tam has been involved in operating and testing police speed guns since 1999. He tests the force's 50 guns every six months.

At the hearing against Mr Lam in Tsuen Wan Court, Senior Constable Chan Tak-cheung, admitted he had conducted a scope alignment test of the laser gun - to ensure it fell on the right target - from 60 metres, rather than the required 200 metres, 'for the sake of convenience'.

In another test the officer recorded the wrong distance.

In an English court, either of those two reasons would have had the case thrown out, Dr Clark said.

Professor Tam disagreed. 'During the scope alignment test, there is no requirement on the distance of a target ... he simply needs to point the gun at various targets at different distances and confirm the gun can receive different signals during the test.'

Professor Tam said the laser gun was easy to use and very accurate. 'The laser gun operator needs to aim it at a vehicle's licence plate at an angle no more than 5 degrees. He should also stand no less than 50 metres and no more than 500 metres from the vehicle.

'The diameter of the laser beam hitting a target 100 metres away would be as large as 30cm, and 60cm for a target 200 metres away, so it is very easy to hit a licence plate.

James To Kun-sun, legislator for the security sector, said: 'Police should explain whether it is a defect of the machine or something wrong with the operation, or else everyone would challenge the capability of the laser gun.'

Mr To said he and fellow legislator Andrew Cheng Kar-foo were liaising with the government to set up special panel meeting in the Legislative Council over the case.

The police have set up a taskforce to review the operation of laser guns.