A system and method for speed measurement of a mobile device, using an speed sensor such as a satellite navigation system or an accelerometer, does not need such speed sensor to be continually activated. The system and method make use of a monitor for monitoring at least one motion indicium for indicating that the speed of the mobile device is above an initial threshold, including but not limited to: monitoring the Doppler shift measurements of a timing signal; monitoring the timing advance of a cellular radio signal; monitoring the number of base stations visible to the mobile device; monitoring the frequency of base station handover, however determined; monitoring the level crossings and/or the autocorrelation of an RSSI signal; and/or triangulation of the position of the mobile device. When the monitor determines that the mobile device may be travelling in excess of a threshold speed, a speed sensor is activated to confirm the indication. If the speed sensor determines that the mobile device is travelling at a speed below a second threshold, the speed sensor is deactivated.