The Idea is to place Cones or Barriers in the road and delete them when they aren't needed any more.
This way PD or EMS, can close a road, or redirect traffic, close a crime scene, etc.
by the use of /placeroadblocks or /prb a menu with all the roadblocks would be show in CEF, You select the roadblock and it will appear in front of you, but I will not spawn on public until you press an X key. This roadblock would be moving with you until you place it on the ground with the X key. So you can place the roadblock at a desire spot.
I took the liberty of making a list of the object names of roadblocks and cones for this usage:
Example:
Source: Link