Obstacles that force a rider to dismount and run with their bike or to "bunny hop" include banks too steep to ride up, steps, sand pits and plank barriers. Besides the start/finish area, these obstacles may be placed anywhere on the course that the race director desires. Several race directors have tried to limit bunny hopping for safety reasons by placing barriers in pairs or in triple (although under UCI rule no more than two barriers can appear in succession). The regulation height for a barrier is 40 cm although this is treated as a maximum at smaller events. Plank barriers seem to be more common in the US than in Europe, and UCI regulations only permit one section of them on the course. Tree logs of maximum diameter 40 cm are sometimes used as barriers in North America.