From Halopedia, the Halo wiki
The traffic cone is a cone-shaped item placed on roads to restrict vehicle access.
In Halo 3 it is on many human-based maps, such as The Pit and Foundry (Heroic Map pack). These cones are usually found around fusion coils. If the fusion coils are shot, or a grenade is thrown at them, or they are stuck with a Anskum-pattern plasma grenade/spike grenade, the traffic cone will shoot across the map at great speed. It is possible to be splattered by a traffic cone. They also appear several times in campaign.
List of appearances