NASHVILLE, Tenn. -- U.S. police officers have heard of the 21 foot rule -- That is, if an attacker is 21 feet away and the attacker suddenly attacks with a weapon, the officer has 1.5 seconds to react, and even then the officer is injured half the time.

Sgt. Mark Chesnut, who was shot Thursday during a traffic stop, was wearing a police revolver with a snap-on top. Chesnut was seated in his car entering information on the in-car computer.

It's much slower pulling this gun from a seated position, as it must be rocked out of the holster.

Veteran police officers said that even if Chesnut had seen the assailant leap from the car, run down and start shooting, he would have had two seconds maximum to defend himself.

For decades, Nashville police officers had partners -- two per squad car. That continued on high-risk shifts well into the 1980s. The idea of returning to two officers per car is not being considered.

"I don't think that there's any reason to lose sight of the fact that thousands upon thousands of these things happen every day, and everybody leaves fine," said Metro Police Chief Ronal Serpas.