Because drinking isn't dangerous, and driving isn't dangerous – at least, not in comparison to the single activity "drinking and driving" – the two words are treated as a singular unit.

This might happen a lot when we combine two or more elements in a sentence:

My favorite drinks are gin and tonic.

This could mean I have two favorite drinks: gin is a favorite drink, and so is tonic.

My favorite drink is gin and tonic.

This means that a gin-and-tonic is my favorite drink.

Because drinking isn't dangerous, and driving isn't dangerous – at least, not in comparison to the single activity "drinking and driving" – the two words are treated as a single unit.

This might happen a lot when we combine two or more elements in a sentence:

My favorite drinks are gin and tonic.

This could mean I have two favorite drinks: gin is a favorite drink, and so is tonic.

My favorite drink is gin and tonic.

This means that a gin-and-tonic is my favorite drink.

Because drinking isn't dangerous, and driving isn't dangerous – at least, not in comparison to the single activity "drinking and driving" – so the two words are treated as a singular unit.

This might happen a lot when we combine two or more elements in a sentence:

My favorite drinks are gin and tonic.

This could mean I have two favorite drinks: gin is a favorite drink, and so is tonic.

My favorite drink is gin and tonic.

This means that a gin-and-tonic is my favorite drink.

Because drinking isn't dangerous, and driving isn't dangerous – at least, not in comparison to the single activity "drinking and driving" – the two words are treated as a singular unit.

This might happen a lot when we combine two or more elements in a sentence:

My favorite drinks are gin and tonic.

This could mean I have two favorite drinks: gin is a favorite drink, and so is tonic.

My favorite drink is gin and tonic.

This means that a gin-and-tonic is my favorite drink.