In Japan, people put their hazards on to say thank you when someone has let them in.
People put on their four-way flashers to say thank you to other drivers who have let them go first.
この場合、次のような言い方ができますよ。
ーIn Japan, people put their hazards on to say thank you when someone has let them in.
「日本では入れてもらったらハザードランプをつけてありがとうと言います」
to put one's hazards on「ハザードをつける」
ーPeople put on their four-way flashers to say thank you to other drivers who have let them go first.
「ハザードランプをつけて先に行かせてくれた他のドライバーにありがとうと言います」
to put on their four-way flashers でも「ハザードランプをつける」ことを言えます。
ご参考まで！
カナダ 
