Roadblock
From Halopedia, the Halo wiki
Roadblocks may be seen many times in both the Halo 2 and Halo 3 campaigns as well as Halo 3's multiplayer.
Covenant
The Covenant employ Roadblocks to block access to enemy troops and vehicles that might try to attack or bypass a fortified position. These roadblocks are small flat platforms that are placed on the ground, the top section is then pushed up on a hinge by an anti-gravity beam. The gravity beam can be disrupted by sustained weapons fire, or a small explosion, nullifying the roadblock's use by lowering the raised platform and allowing passage. Many Covenant troops will also use the roadblocks as a cover when being fired upon. These roadblocks can also be used as a ramp for a vehicle to jump over enemy fortifications. This will only work if you face the direction that it slopes up.
These can be seen around the levels Outskirts and Uprising.
In Halo 3, the roadblocks are much smaller and have three connected plates with a wide base. It can be destroyed but it will can stop vehicles. It appears in the levels Tsavo Highway and The Ark
Human
Human roadblocks are small presumably concrete walls. They may appear on a few campaign levels in Halo 3, such as Tsavo Highway. They appear in no multiplayer maps automatically, but they can be placed with Forge.
