Felonies are not usually witnessed by police officers. Sometimes, but not usually. Traffic stops can lead to more than tickets for simple traffic violations and police know that. It is called "good police work." The vehicle you are driving or riding in is pulled over. Then the Officer sees, hears, or smells something and the next thing you know one or all of the occupants of the vehicle are handcuffed and seated in the back of police cars to face eventual felony charges.
Police can make the initial stop for all sorts of reasons like:
Following too closely as you can read by clicking here, 2009-Ohio-5199.
(The defendant ended up charged with possession and trafficking of drugs after her friend was following another vehicle too closely, the passenger and driver gave the officer different answers to questions, the passenger was intoxicated, a K-9 unit was called out, the dog alerted on the vehicle giving the officer probable cause to search, etc. It all started with following too closely.)
Failing to use your turn signal as you can read by clicking here 2009-Ohio-4897, or here 2009-Ohio-994, when either turning or switching lanes. In 2009-Ohio-559 the failure to use the turn signal when turning into someone's driveway led to a canine sniff and an alert on materials to make meth which ultimately led to a three year prison term.