An improved accuracy is ensured for the calculation of the travel distance of a bulldozer by obtaining the accurate vehicle speed and travel direction of the bulldozer. To this end, arrangement is made such that a check is made to determine whether actual tractive force F exerted on the blade exceeds a predetermined shoe slip threshold value F.sub.0 ; switching between vehicle speed detectors (between a Doppler sensor and Sprocket rotating speed detector) is made according to the determination so that suitable one of the vehicle speed detectors detects the vehicle speed of the bulldozer; and the vehicle speed detected by the selected vehicle speed detector is integrated, whereby the travel distance of the bulldozer is calculated.