should inable member states desire to collect fees from road users certain limited external costs that the polluter pay'es principle can finally introduced in the area of 
The directive should enable Member States if they so desire to collect fees from road users for certain (limited) external costs, so that the 'polluter pays' principle can finally be introduced in the area of road transport.
should enable Member States desire to collect fees from road users certain (limited) external costs, that the 'polluter pays' principle can finally introduced in the area of road transport.