Close down the car sales or at least stop them from importing stocks without orders.
Let the people who want to buy, order and get a
1. Brand new car from the agents
2. Used car from dealers for a commission like the individual imports today.
That way government will be able to control the imports and would allow them to track the income of people more easily.
And for the traffic problem there needs to be a way to encourage people to use more public transport. To do that,
1. if you want to solve it with buses, you will need to add more buses for the peak hours, increase the quality of the service and bring in stricter rules for the number of passengers. This would solve the problem but might not be economically sound.
2. Improve the train service on the main lines and have a good inter-connecting bus system for the short hauls.
Once they have a solution, just put a high city entry tax that would discourage people from using personal transports.
But just like the liquor/cigarette problem, the govt would take a big tax hit if the petrol consumption reduces.
And in Sri Lanka, the govts never ever take hard unpopular decisions, so all this is pointless as nothing will ever be implemented.