A roadblock refers to an ad buy by which all ads on the page are dedicated to the advertiser. Thus, the share of voices on the page are 100%.
More often, roadblocks include two ad placements and sometimes a third one.
Roadblock may be bought on a stand alone basis or be a complement for some special ad formats. For instance, interstitial ads or floating ads are often associated with a roadblock after their disappearance.
Roadblocks may reduce clutter perception and favor the notice of advertisers.
A roadblock is usually sold at a lower price than the overall price of concerned ad placements.
On some rare occasions, the different elements of the roadblock can interact. It is then a "false" roadblock and a rich media ad (see example below).
Examples of roadblock:
A "false" roadblock: