"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."