UCI Regulations - Track Races
3.2.005 Riders may carry no object on them or on their bicycles that could drop onto the track. They may not bear or use on the track any music player or radio communication system. In addition, any electronic device with display (for instance speedometer or powermeter) must be hidden to that it cannot be read by the riders.
Acceptable mounting to hide the device from view is under the saddle. Tape over the display is not acceptable since theoretically it could be removed. Racers are allowed a coach on the apron of the track while they are racing. Coaches are allowed to provide lap times. Some coaches use a step command where they walk toward or away from the oncoming racer to signal to either increase, decrease or hold pace and others yell out the lap times.