The roadside test isn't the only thing that gets considered when making an arrest. The choice to even administer the test has to have some basis, like smelling an odor of an alcoholic beverage or other intoxicant or seeing symptoms of drug use. There would also have to be some driving behavior that was a violation of traffic law or was irregular enough to warrant investigating the cause. The roadside tests are also of differing types, physical and mental and really only show if the driver can perform basic tasks. Most of us in law enforcement have had the occassion to stop a person who looked like they were driving drunk but were instead in the middle of a diabetic or other medical emergency. We get trained to recognize those situations and respond appropriately. In those rare instances, I've never had someone complain that their car got towed because they could not safely operate it.