Like I said , there are some bad apples.......

Also, traffic doesn't usually get involved in detective work and vice versa. If you start giving mixed directives to those branches they could easily wipe out a case being built against an alledged criminal. You know the example, rookie cop goes in to bust a deal only to find out the "dealer" is from another law enforcement group and the buyer (the intended target) walks due to a proceedural technicality in the law.

Criminals are organized so well these days that by the time your pullover cop could get to the dealer, the dealer would have been gone, since he got a 5-0 call on his cell phone when the marked car approached the end of the block. The stash would've been given to a minor or tossed.