Quote: Most of the time they don't arrest you just because they suspect you may be under the influence of a drug that most likely does not affect driving negatively. Unless there is some evidence out there in the open, you won't go to jail for having taken any drug or being under the influence of a prescription med.

Keep believing that if you would like. It is an officers responsability to detain or arrest anyone operating a motor vehicle under the influence. They don't need to observe any evidence other than the persons inability to operate the vehicle in a safe manner. Perhaps these statistics might enlighten you.
Approximately 1.5 million drivers were arrested in 1999 for driving under the influence of alcohol or narcotics. This is an arrest rate of 1 for every 121 licensed drivers in the United States (2000 data not yet available).
[NHTSA, NCSA 2000 ]

In 2000, about 1,400 fatalities occurred in crashes involving 1,260 alcohol-impaired or intoxicated drivers who had at least one previous DUI or DWI conviction - about 8.5 percent of all alcohol-related traffic fatalities.
[NHTSA, NCSA 2000]

Among the 5.3 million convicted offenders under the jurisdiction of corrections agencies in 1996, nearly 2 million, or about 36 percent, were estimated to have been drinking at the time of the offense.
[Greenfield, 1998]

According to the Bureau of Justice Statistics, 40 percent of persons convicted of homicide and 25 percent of victims had been drinking at the time of the event.
[Hingson et al, October 2001]

One arrest is made for driving under the influence for every 772 episodes of driving within two hours of drinking and for every 88 episodes of driving over the legal limit in the U.S.

Take care. Billylll