Prohibited Cargo and Restrictions
What you cannot legally haul in a double or triple combination.
Endorsement: Doubles / Triples (T) · Source: FMCSA CDL Manual (public domain)
Federal and state regulations prohibit certain hazardous materials in doubles and triples because the additional combination length and the dolly couplings increase the risk of release. Class 1 explosives Division 1.1, 1.2, and 1.3 generally cannot be transported in any double or triple combination on most highways. Many state regulations prohibit additional hazmat classes in multi-trailer configurations, and drivers are responsible for knowing the rules of every state on their route.
Length and weight restrictions also vary by state. The federal interstate standard allows doubles up to 28 feet per trailer (often called twin 28s) on the National Network of highways, but some states permit longer combinations on specific routes (Rocky Mountain Doubles, Turnpike Doubles, and triples on certain western interstates). Combinations longer than the standard twin 28 are rarely permitted east of the Mississippi River, and triples are restricted to specific routes in a smaller set of western states.
Weather restrictions can also apply. Several states close their highways to doubles and triples when wind speeds exceed posted limits, because the long combinations are especially vulnerable to crosswind-induced trailer sway and tipover. Drivers should monitor state DOT alerts and be prepared to drop a trailer or wait out the weather. The CDL Doubles/Triples exam tests these prohibitions in general terms; specific state limits change frequently, but the principle that doubles and triples have route, length, and weather restrictions beyond those for singles is consistent across all states.
Key terms to memorize
- converter dolly
- pintle hook
- safety chains
- crack the whip
- rear trailer
- pup trailer
Other Doubles / Triples (T) topics
- Coupling Order for Doubles and Triples — Which trailer goes where, and why the heaviest goes first.
- Converter Dollies — The two-axle dolly that turns a semitrailer into the second unit of a double.
- Inspection of Doubles and Triples — Walking the full length of a double or triple before every trip.
- Handling Doubles and Triples on the Road — Following distance, lane changes, and the crack-the-whip physics.
Test what you learned
Now that you have the Prohibited Cargo and Restrictions material in your head, drill the Doubles / Triples (T) practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Doubles / Triples (T) test for your jurisdiction.