What must tread wear bar indicators not do during operation?

The appropriate characteristic of tread wear bar indicators during operation is that they should not contact the road in major grooves. Tread wear bar indicators are designed to reveal the level of tread on a tire visually; they indicate when the tread is worn down to a critical level where it may compromise traction and safety. If these indicators were to make contact with the road surface, it would signify that the tire tread is excessively worn and unsafe for driving conditions. This contact could lead to reduced braking performance and increased risk of hydroplaning, which can be dangerous. Therefore, the function of the tread wear indicators is to alert drivers before this critical point is reached, emphasizing the importance of maintaining adequate tire tread depth for safe vehicle operation.