Many cyclists may be unaware that a ban on riding against the flow of traffic will take effect by the end of the year to curb an increasing number of bicycle-related accidents, police warned Friday.

Under the revised Road Traffic Law, which will take effect by Dec. 13, cyclists who don't keep to the left-hand side of the road may face up to 30 days in prison or a fine of ¥20,000, the police said.

Cyclists are currently allowed to ride in narrow side lanes also used by pedestrians in the absence of sidewalks, and they can ride in both directions. However, the revision made June 14 stipulates that bicyclists must now stay on the left side of the road at all times.

Cyclists will still be allowed to ride in either direction on sidewalks that permit cyclists.

According to National Police Agency data, 3,956 cyclists nationwide were given warnings in 2011, including 17 that reportedly led to fines.