So could they charge you with a DUI if you failed EITHER the motor skills tests or the Breathalyzer?
Technically the whole DUI thing is a loaded 'question'. Most out on the weekend after 9pm have been drinking, many would be over the limits. Obviously this is the country as a whole and there are going to be exceptions.
The field sides are enough for a cop to make an arrest. Without a BAL though it's an easy defense in court. Blow above the limit and the way the law works is you are pretty much fucked.
The thing about the field sides is almost everyone looks 'drunk' doing them and they are filmed I think by all police now.
Also the police report write up will be very biased and weighted heavily to reflect a much worse portrayal of the incident. This is where it's the cop's word versus your's.
A typical write up:"Was behind driver approximately 1mile prior to stop. Driver kept stopping abruptly (confused/disoriented?), failed to maintain own lane (several times). As I approached the vehicle, driver looked confused, glassy eyed and had a reddish complexion.
Smell of alcohol was noted and driver noted they had been drinking (3+ drinks), asked driver to complete FST. Driver's speech was slurred and had difficulty following conversation. Had to repeat statements many times.
Driver had difficulty exiting vehicle, almost fell...driver had hard time understanding instructions and maintaining an upright posture (kept leaning left to right)."....
these always look a mess.