Roadblocks may be seen many times in the Halo 2, Halo 3, and Halo 3: ODST campaigns as well as Halo 3's multiplayer.
Covenant[]
A Covenant Deployable Roadblock in Halo 2.
The Covenant employ Roadblocks to block access to enemy troops and 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 antigravity 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 can still stop, especially Wraiths. To get a wraith past one of these roadblocks, you must either use its mortar or the boost. Covenant Roadblocks appear in the levels Tsavo Highway and The Ark.
They do not currently appear on any multiplayer maps by default, except The Pit (which is on the other side of the fence). The roadblock can be added on with Forge.
Human[]
Human roadblocks are small concrete walls. They may appear on a few campaign levels in Halo 3, such as Tsavo Highway or at the beginning of the Storm. They don't appear in any multiplayer maps by default, but they can be placed with Forge. They can be damaged, but not completely destroyed.
A Covenant roadblock in Halo 3.