Why must some hazardous materials vehicles avoid certain tunnels and bridges, or follow designated routes?

Some hazmat vehicles must AVOID certain TUNNELS, BRIDGES, and densely populated areas, or follow DESIGNATED ROUTES, because a hazmat incident (fire, explosion, toxic release) in a CONFINED space like a tunnel, or on critical infrastructure like a major bridge, could be CATASTROPHIC — endangering many people and damaging vital infrastructure. CDL hazmat. ROUTE RESTRICTIONS minimize the risk to the public, infrastructure, and the environment. Drivers must KNOW and FOLLOW applicable federal, state, and local route restrictions for the materials they carry (some materials, like explosives or certain toxics, have stricter rules). Signs may indicate hazmat restrictions. Knowing why route restrictions exist (minimizing catastrophic risk in confined/critical areas) is commonly tested.