Most police agencies in California now use a portable breath testing machine prior to making a DUI / Drunk Driving arrest. In California, if the driver is older than 21 and not on DUI / Drunk Driving probation, then the portable pre-arrest breath test machine is completely voluntary. The investigating police officer is required by law to advise the person being investigated for DUI / Drunk Driving that the pre-arrest portable breath test is completely voluntary. For years, I have been stating during many radio station interviews that post arrest breath testing machines are not accurate in determining a person's alcohol level in their blood. The portable pre-arrest breath testing machine is even less accurate than the post arrest breath test machine used by many police agencies at either their police stations or jails.