When Massachusetts police officers suspect that a driver is intoxicated by alcohol, they often ask the motorist to perform a standard field sobriety test. If the driver has trouble walking in a straight line or reciting the alphabet, the police officer may have a reasonable suspicion that the driver is drunk. A breath test or a blood test can also be used to measure the driver's blood-alcohol content.
Because there is no good test for marijuana intoxication, police officers have a lot more difficulty proving that drivers are stoned. A Massachusetts public defender has asked the state's Supreme Judicial Court to disallow the use of field sobriety tests as evidence for marijuana intoxication. According to the attorney, there is no proof that flunking a field sobriety test indicates marijuana intoxication.