Criminal procedure discussion 9

 State Trooper Jones follows Mike Smith, driving his car down the highway at normal speed. After three miles of being followed, Smith changes lanes without signaling. Trooper Jones immediately turns on the lights and sirens and signals Smith to pull over. Smith does so.

The trooper, in full trooper regalia, walks up to the driver's window, leans in, and asks, ominously, "do you know why I pulled you over?" Smith says no. Jones says, "you changed lanes without signaling". Jones, nervously, gulps and apologizes. "License and registration, sir" says Jones. Smith complies. While examining the documents, Trooper Jones detects the odor of burnt marijuana (in this jurisdiction it is still illegal to possess or use marijuana) from inside the car. Trooper Jones asks Smith " have you been smoking weed in your car just now?" Smith stammers "uh-um-ah". Trooper Jones tells Smith to get out of the car and quickly puts him in handcuffs. Trooper Jones then asks Smith is there any more drugs in the car? Smith replies "yes, in the trunk". Trooper Jones rummages through the trunk and finds a bag of cocaine, about four ounces. 

Trooper Jones then asks Smith if he is being paid to transport the drugs, a defeated Smith replies "yes".

What right or element of criminal procedure is involved here? If so, what triggers the right/procedure?

Can the admission Smith gave about the drugs in the car be used in court against Smith?

What about the admission of drug running?

Should they be admissible?

Is there a case about questioning that is on point here? What is it? What did it say? 

Have you heard of or read about any similar cases?