on the spur of the moment

Miyuki MiyabeのAll She Was Worthを読んでいます。
The chair in the store was small and its legs were wobbly. Not one other customer came in while he waited. It was too drafty and cold just to sit there. On the spur of the moment, he stepped outside and went to a public phone down the street where he dialed Mizoguchi's number. A woman answered. Must have been that literary Ms. Sawagi.
馬に乗ったことはありませんが "spur" に「拍車をかける」意味があるのは知っています。しかし、"on the spur of the moment" の意味が分かりません。慣用句として使われている様だ。辞書を引きます。
・Oxford English Dictionary: On impulse; without planning in advance.: I don't generally do things on the spur of the moment.
・Collins Dictionary: If you do something on the spur of the moment, you do it suddenly, without planning it beforehand.: It wasn't a spur-of-the-moment decision. We discussed it in detail beforehand.

・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: suddenly; spontaneously. We decided to go on the spur of the moment. I had to leave town on the spur of the moment.
拍車をかけたように突然何かを始めた時に使うのですね。