How does cannabis affect driving?

Cannabis is a depressant drug that may also have some hallucinogenic effects.

The depressant effects slow down the activity of the central nervous system, including the brain. The hallucinogenic effects can distort perceptions of the world, making people see or hear things that don't exist.

Find out more about cannabis.

Using cannabis puts drivers at a similar risk as that of a driver with a blood alcohol concentration (BAC) above 0.05 (Arrive Alive 2009).

The effects of cannabis can be different for each person. They may be influenced by:

• how much is used. Larger amounts can produce different effects.
• how  much THC (delta-9 tetrahydrocannabinol) it contains.
• how it is used. For example, when cannabis is smoked the effects are experienced very quickly and may last up to 5 hours. When cannabis is eaten, the onset of effects can be delayed by about 60–90 minutes and can last up to 24 hours.
• the mind and body of the person using it.  The mental and emotional state can alter the drug's effects.

These factors make it difficult to predict exactly, in what way, and for how long, cannabis will affect driving ability.

As a general guide, cannabis can cause:

• reduced coordination
• slower reaction times
• slower information processing
• confusion
• changes in vision, heaing, and time and space perception.

The "hangover" effects can last for several hours and can also affect driving ability.

A person who has been using cannabis may think that, if they are especially careful, they will be able to drive safely. However, the cannabis may have affected their view and experience of reality, and their judgement. Their actions and responses may be quite different to what is actually needed, but they may not be aware of how much their driving skills have been affected.

As a rough guide, the devices used in  Victoria's random roadside drug testing can detect the THC (Delta-9 tetrahydrocannabinol) in cannabis for several hours after the last use.

Tips for driving safely

If you have used cannabis, the safest option is not to drive.

Especially avoid driving late at night or early in the morning when natural tiredness may increase the sedating effects of cannabis. 

Find more information for drivers.

Arrive Alive 2008 "Random roadside drug testing" (accessed 15 June 2011)

Last updated: 27 April 2012