I personally would write, though there are many variations,
"Motorbike DRIVERS mustn't <strike><font style="background-color: yellow;">run</font></strike> <font style="background-color: yellow;">travel at</font> more than 40km/h in the city" <sub></sub><sup></sup><strike></strike>
I would just say "the speed limit for motorbikes is 40 km per hour" in the city"
Have a nice day.
I think they musnt travel 40 km/h for safety