One of the early system's design limitations was the inability to enforce speeds below 40 km/h. Several accidents at train stations and railroad yards occurred because the driver failed to stop at a red signal and ATB did not intervene, as it had already enforced a speed below 40 km/h. Strange as this limitation may seem, there was a reason for it. The main purpose of ATB was to prevent overspeeding and failing to respond to yellow signals, at the time it was deemed unlikely a driver would miss a red signal when traveling below 40 km/h and expecting one. It is seen as a problem now as numerous accidents have occurred and safety regulations became stricter over time.