Learn English – Do you know another word for the most commonly used

Complacency is the correct word to describe the act of the motorcyclist. Chambers defines complacent as:

• Showing satisfaction, esp when self-satisfied, usu with insufficient regard to problems, dangers, etc

The following sentence can describe the motorist aptly.

• The complacent motorist lost his life in a freak show of bravado.

The commonly-used idiom is to "kill two birds with one stone".

to succeed in achieving two things in a single action: I killed two birds with one stone and picked the kids up on the way to the train station.