Final answer:
The incorrect statement about long combination vehicles is that they are allowed on all highways. They have specific highway restrictions due to safety and infrastructure concerns.
Step-by-step explanation:
The statement about long combination vehicles that is incorrect is c) They are allowed on all highways. These types of vehicles require special training and licensing due to their complexity and the safety risks they pose. It is true that they are colloquially referred to as road trains, particularly in regions like Australia where they are common. These vehicles do have specific weight and length restrictions, which vary depending on the country or even the state or province within a country. Therefore, they are not permitted on all highways. Restrictions may be based on the infrastructure capabilities such as bridge capacities and road design, as well as safety considerations for public highways.