"You're not meant to ride [bicycles](https://eikaiwa.dmm.com/uknow/questions/29830/) side by side."
"you're not meant to ~" 「〜してはいけません」
"you're not allowed to ~" と "you're not supposed to ~" も同じように使えます。
"to ride bicycles side by side" 「自転車で並走」"to ride bicycles alongside each other" も言えます。
"According to Japanese traffic law, you're not supposed to ride bicycles side by side on the road. however many people don't follow the rules and ride their bikes alongside each other while talking."
There are roads in Japan where you're not allowed to cycle next to another cyclist.
この場合、次のような言い方ができますよ。
ーThere are roads in Japan where you're not allowed to cycle next to another cyclist.
「日本の道路には自転車で並走してはいけないところがある」
to cycle next to another cyclist で「並走して自転車に乗る」ことを言えます。
ーBut a lot of people ignore this rule, and just cycle down the street side by side chatting with their friends.
「でも多くの人がこのルールを無視して、友達とおしゃべりしながら並走して自転車に乗っています」
ご参考まで！
オーストラリア 
カナダ 
