Method and arrangement for controlling a subsystem of a vehicle dependent upon a sensed level of driver inattentiveness to vehicle driving tasks. A variable characteristic is measured, on a substantially real-time basis, which correlates to the driver's inattentiveness. The level of inattentiveness is assessed based at least in part on the measurement. The performance of a subsystem of the vehicle, such as cruise control or lane keeping support, is tailored, based thereupon, to assure that behavior of the vehicle appropriately matches the driver's present level of inattentiveness. The subsystem's operation is controlled in an effort to avoid or prevent the establishment of driving conditions that become inherently more dangerous as the driver's level of inattentiveness increases.