In discussions of the road trains of 6-8 vehicles that are kept in close formation for higher fuel efficiency and as a step toward robotic driving there is concern that the long line of vehicles would present issues for vehicles entering and exiting the highway.
In this 46 page pdf presentation on Truck Weight Reform Future Truck Loadings there is a discussion of existing long trucks
State regulations vary but legal permission for 65 foot long trucks is common. Certain sections of highway and other countries allow longer trucks up to 110 feet in some cases.
The road trains of 6-8 vehicles should be at least as safe as the long trucks. Signage and electronic indicators transmitted from the lead bus or truck could provide warning to other drivers of the road train behind. Electronic systems allow more adaptability and flexibility to increase the separation gap if needed in a middle section for safety and courtesy to other cars on the road.
Long Truck Safety
Long Truck Fuel Economy