Kevin Clinton, head of road safety at RoSpa, said: "Even if they are legal to ride on the road we are concerned that their small size will make it harder for drivers to see the riders - especially in traffic and at junctions." He added: "The small bikes are more appropriate for off-road use, but even then they need to be used in controlled environments so riders don't endanger others. We are also concerned that users will not be trained to use this specific type of bike."