When is it illegal to drive on the left half of the roadway?

Driving on the left half of the roadway is illegal within 100 feet of a railroad crossing because this regulation is designed to ensure safety for both motorists and individuals using the railroad. The goal is to prevent dangerous situations where vehicles might obstruct the view of an oncoming train or where the vehicle could be at risk of being struck by a train. The law mandates that drivers must remain in their designated lanes to promote safe traffic flow, particularly in critical areas such as those near railroad crossings.

In contrast, the other situations presented in the choices do not inherently restrict driving on the left half of the roadway. For example, while driving under the influence is illegal, it pertains to the driver's state rather than specific roadway rules. Likewise, residential areas do not have a blanket rule prohibiting left-side driving, nor do stop signs dictate lane usage unless specified by local traffic regulations. This context clarifies why driving on the left is illegal specifically near railroad crossings.