What action is prohibited when driving on the highway?

Merging into traffic without a turn signal is prohibited when driving on the highway because it poses a significant safety risk to both the driver and other road users. Turn signals are essential to communicate a driver's intentions to other vehicles. When a driver merges without using a signal, other drivers may not be aware of the upcoming lane change, increasing the likelihood of collisions. Proper signal use allows for smoother traffic flow and enables other drivers to react accordingly, contributing to overall road safety.

Using cruise control is a legal action, provided the driver remains attentive and is in a suitable driving condition. Changing lanes without looking can result in dangerous situations but is often more of a driver responsibility rather than a strictly prohibited action, though it is still extremely unsafe. Driving in the left lane can be permissible depending on the state laws and the circumstances, particularly regarding passing slower traffic.