Which modifications are not allowed when driving a commercial vehicle on highways?

When considering modifications to commercial vehicles on highways, it's important to recognize the regulations governing vehicle safety and performance. Studded rims are often prohibited because they can cause significant damage to road surfaces and reduce the grip of a vehicle on various types of pavement, particularly in warmer conditions.

Heavy-duty axles, enhanced headlights, and extra seating, while potentially subject to regulation, can generally be modified within certain guidelines to ensure that the vehicle can operate safely and effectively. However, the addition of studded rims is typically restricted due to the negative implications for road infrastructure and overall safety, which is why this modification is often flagged as not allowed on highways.