Published by Australian Academy of Science
The dope on drug-impaired driving Box 1 | The Standardised Field Sobriety Test (SFST)

The first of three tasks conducted under the SFST is the ‘horizontal gaze nystagmus test’. The police officer asks the driver to look at an object (such as a torch, pencil or finger) held about 30 centimetres in front of the driver’s eyes above eye-level. Drivers are then asked to track the object with their eyes as it is moved slowly to the side. This test is designed to evaluate the degree to which the driver is suffering from nystagmus – the involuntary jerking of the eye that occurs when the eyes look to the side – which commonly occurs at a smaller angle from straight ahead under the influence of alcohol. For each eye the officer looks for three clues including a lack of ‘smooth pursuit’ (jerkiness in the eye as it follows the object) and the onset of nystagmus prior to 45 degrees.

The second task is the ‘walk-and-turn’, which itself comprises two parts. In the first part, the driver must stand heel-to-toe while the police officer explains the test. In the second part, he or she must take nine heel-to-toe steps in a straight line, turn around and repeat the steps in the opposite direction. Officers are trained to score the performance against eight clues, including an inability to follow instructions, over-balancing and taking an incorrect number of steps. Drivers who score two or more clues are classified as having a BAC, or drug-induced BAC equivalent, of over 0.01 per cent.

In the third task, the ‘one leg stand’, drivers are asked to stand with their arms at their sides and to hold one leg at least 15 centimetres above the ground for 30 seconds, counting out the number of seconds as they do so. Officers look for four clues including swaying and putting the foot down.

Related sites

• Appendix A: Standarized field sobriety testing (US Department of Transportation)