Drug Rehabilitation that works

I was watching COPS one day and a police officer busted someone for driving under the influence of a controlled substance (methamphetamine).

Here are my questions:

What driving patterns suggest that the driver is driving under the influence of a powerful drug like methamphetamine?

What physical characteristics (heart rate, eye movement, speech) suggest that the driver has been using methamphetamine, possibly for the first time?

Yes, I know that drugs affect everyone differently, but please tell me about common symptoms.

i have heard several clients mention this side effect.

Also, can someone very briefly explain the threshold and interaction effect as it relates to developmental psych?