A number-of-operations information storage unit (113) stores number-of-operations information in which a plurality of conditions of the vehicle is defined and number of acceleration operations and number of deceleration operations performed by a vehicle are described for each condition defined. A correction speed specifying unit (109) detects a current condition of the vehicle, when the vehicle travels at a constant speed, extracts number of acceleration operations and number of deceleration operations described for a condition corresponding to the detected current condition of the vehicle from the number-of-operations information, compares the extracted number of acceleration operations with the extracted number of deceleration operations, specifies a correction speed higher than a set speed for a constant-speed travel if the number of acceleration operations is larger than the number of deceleration operations, and specifies a correction speed lower than the set speed if the number of deceleration operations is larger than the number of acceleration operations. A constant-speed travel control unit (108) makes the vehicle travel at a constant speed based on the correction speed specified by the correction speed specifying unit (109).

A prime mover control device of a work vehicle equipped with a torque converter includes: a speed ratio calculation unit that calculates a speed ratio of the torque converter; and a rotational speed limit unit that, when a speed ratio falls within a preset speed ratio range, limits a maximum rotational speed of the prime mover to be lower as compared with a maximum rotational speed set when a speed ratio falls without the preset speed ratio range. When a state in which the speed ratio calculated by the speed ratio calculation unit falls within the preset speed ratio range is maintained for a predetermined time period, the rotational speed limit unit changes a maximum rotational speed of the prime mover to a higher maximum rotational speed than the limited maximum rotational speed limited to be lower by the rotational speed limit unit.

An upper-limit vehicle speed should be prevented from being set to speed different from an intention of a driver. When it is detected that a speed limit is changed, an upper-limit vehicle speed setting ECU temporarily switches a control mode from a normal mode to an inquiry mode. An ASL operation unit serves a function for a change operation of upper-limit vehicle speed in the normal mode, and serves a function for a reply operation about whether the speed limit is accepted in the inquiry mode. When the ASL operation unit is operated in the normal mode, the upper-limit vehicle speed setting ECU does not switch the control mode to the inquiry mode, even if a change of a speed limit is detected, until a certain period of time (a certain period during which a mode switching prohibition period is set) has passed since the operation.

A throttle control system and method for use with a vehicle. The throttle control system receives an input voltage from a vehicle throttle controller. The throttle control system prepares a throttle signal and provides the throttle signal to the throttle to force deceleration to a selected speed or constrain acceleration to a selected value. Preparing the throttle signal may follow detection of maximum threshold values of speed or input voltage. The maximum threshold values may be specific to the location of the vehicle. The throttle signal and the maximum threshold values may be defined with reference to the input voltage, vehicle speed, vehicle location, or other parameters. The throttle signal may force deceleration or constrain acceleration incrementally to mitigate loss of vehicle throttle controller responsiveness to driver commands. The throttle signal may be an output voltage provided to a vehicle electronic control module.