N4
どんどん
Progressively, Rapidly increasing, More and more
バスの運転手がどんどんと減っている。
The number of bus drivers is rapidly decreasing.
Explanation
「どんどん」 has 2 uses
1Rapidly, one after another
Things move fast and keep going, often with 「〜くなる」, 「〜になる」, 「増える」, 「減る」: 「どんどん寒くなる」 it keeps getting colder.
どんどん暑くなる。
It will rapidly get warmer and warmer.
バスの運転手がどんどんと減っている。
The number of bus drivers is rapidly decreasing.
2Go ahead, freely
Tells someone not to hold back: 「どんどん食べてね」 eat as much as you like, 「どんどん聞いてください」 ask as much as you want.
どんどん頼んでね！
Order as much as you like!
車の運転免許を取ったので、どんどん車に乗って出かけましょう。
I've got my driver's licence, so let's drive out as much as we like.