A device that stops drivers from falling asleep at the wheel is about to undergo testing at Department of Transport laboratories and could go on sale within 12 months. The system, called Driver Alert, aims to reduce deadly road accidents by 20% - 40% that are caused by tiredness. Airline pilots can also use it to reduce the 30% of all pilot-error accidents that are related to fatigue. Driver Alert is based on a computerized wristband. The device, worn by drivers or pilots, gives out a sound about every four minutes during a car journey. After each sound the driver must respond by squeezing the steering wheel. A sensor in the wristband detects this pressing action and measures the time between the sound the driver's response. Tiredness is directly related to a driver's response time. Usually, a watchful driver would take about 400 milliseconds to respond, but once that falls to more than 500 milliseconds, it suggests that the driver is getting sleepy. In such cases the device gives out more regular and louder sounds, showing that the driver should open a window or stop for a rest. If the driver's response continues to slow down, the sounds become more frequent until a nonstop alarm warns that the driver must stop as soon as possible. The device has been delivered to the department's laboratories for testing. If these tests, scheduled for six months' time, are successful, the makers will bring the product to market within about a year. We can learn form the text that the driver needs to stop for a break when his response time is   _  .
about 400 milliseconds below 500 milliseconds over 500 milliseconds about 4 minutes