OFFICIAL attempts to reduce traffic congestion are likely to cause a mass rush to buy second-hand cars before any new legislation comes in, motor groups warned yesterday. Second-hand cars could soar in price as thousands of people attempt to become car owners before any future legal deadline. Proposals being drawn up by the Government's Transport Branch would limit the number of new car owners by a quota system. But existing car owners will be unaffected, and when they trade in their car for a new one they will not have to bid for one of the quotas. A government source said: ''We would not take away ownership from those who already own cars. The fairest thing to do is to impose the quota on new car ownership.'' A motorist group warned that such a move could lead to a rush to buy any old car as long as it had six months to go on its vehicle examination certificate. Hong Kong Automobile Association spokesman Andrew Windebank said: ''It would seem that one of the ways of ensuring you are able to get your ration of cars is to buy an elderly but roadworthy car now and exchange it for a new one later.'' The chairman of the Motor Traders' Association, Mike Rushworth, said: ''If the Government brings in this regulation there will be a rush to purchase used cars so that everyone has a car when the time comes.'' Transport Secretary Haider Barma confirmed yesterday that a quota system had been discussed by a Transport Branch working group, but recommendations had still to be drawn up. ''It is premature to say that a quota system will definitely be introduced, and certainly at this stage reports that the number of new private cars which could be registered will be limited to 2,000 per annum is incorrect,'' Mr Barma said.