With the increasing number of bicycle riders, there is an increased risk of accidents between bicycles and automobiles. Some accidents are caused by speeding bicycles and the lack of communication between the cyclist and car drivers. To close the communication gap, we propose a method to detect speeding behaviors using a conventional smartphone. Using the smartphone, the cyclist will be able to obtain additional notification with minimum inconvenience and cost. We investigated the fundamental characteristics of accelerometer data with different mounting positions and shock absorber conditions. We found that the rear side position was the best for detecting speeding behaviors.