This description of the Walk and Turn test posted by San Francisco DUI Lawyer.

This test requires a hard, dry, level, non-slipping surface with sufficient room for the suspect to complete nine heel-to-toe steps. It should be administered under relatively safe conditions. This test loses some validity when conducted in certain wind/weather conditions contrary to this criteria. The manual advises that a straight line should be clearly visible on the surface. Conditions must be such that the suspect would be in no danger if he or she were to fall. The subject and the officer's safety should be considered at all times. If these conditions do not exist, the manual advises that the suspects should be asked to perform this test elsewhere or only the HGN test should be used. 

There are some people that this test should not be given. The average sober person would have difficulty under these conditions. People more than sixty five years of age, over fifty pounds overweight, or with any physical impairments that would affect their ability to balance should not be given this test. The officer is trained to take this into account when developing their probable cause to arrest. They should eliminate these conditions being present before they begin the test.