Alcohol at any level impairs driving and our central message always has, and always will be, ‘don’t drink and drive’.
A zero limit would bring some difficulties as people’s response to alcohol varies depending on a range of matters including age, gender, weight, time of day, the time taken to consume alcohol and whether they have eaten. We want to avoid criminalising drivers who may have the remnants of alcohol in their system even though it is quite some time since they had a drink and very little alcohol actually remains in their system.
There is also a risk because there will be cases where an individual would register slightly above zero even when they had not been drinking; diabetes and the use of mouthwash can both cause an above-zero level. It could make enforcement substantially more difficult.