Why are doubles and triples driven on certain routes only?

Doubles and triples — especially LCVs — are restricted to specific routes because the combinations' length, handling, and reduced maneuverability make many roads unsafe. Restricted routes typically exclude tight urban streets, narrow rural roads, low-clearance bridges, and routes with sharp curves or steep grades. Federal regulations and state law together specify which routes allow which combinations. The National Network of highways permits standard doubles (28' + 28') with reasonable connector access; LCVs are limited to the western interstates, certain turnpikes, and approved connector routes. Driving outside the permitted route is a violation and can be dangerous.