they always drive past and i wish i knew some way to make them stay behind, for example with my lights on, also for some reason when i pull cars over it wont allow me to start the traffic stop, the red circle isnt there any help would be appreciated guys 

i think there is a mod for that also with the no circle thing it might be a glitch have you tried reloading scripts?

use vdh police helper and put up a road barrier "CTRL-Z"

I'm hearing there's a 'no overtake mod' somewhere on one of these sites (GPM, GTA4-Mods, LCPDFR, etc, etc), that may help you. Otherwise, like Srgt.Scales said, just use road barriers. Unfortunately, with road barriers, a car pileup usually forms, so you may want to use caution with that. On my traffic stops, the cars always stay behind me, but I want them to go ahead because them stopping just disrupts traffic everywhere.

Are you pressing shift when stopping the car? 

Less overtaking mod: http://www.gta4-mods.com/script/less-overtaking-f15306

You should allow some overtake anyway, find better places to pull people over. Like don't hit em right off the bat.

You could try bravehearts police script you can halt traffic with that

ok thanks guys, and yeah i use shift when pulling them over, the scripts i have all reloaded in, but i will try find better places to pull cars over