A (digital) tachograph is a device fitted to a vehicle that automatically records its speed and distance, together with the driver’s activity selected from a choice of modes: the drive mode is activated automatically when the vehicle is in motion and modern tachograph heads usually default to the other work mode upon coming to rest. The rest and availability modes can be manually selected by the driver whilst stationary.
Regulations vary from country to country
For reasons of public safety, many jurisdictions have limits on the working hours of drivers of certain vehicles, such as buses and trucks. A tachograph can be used to monitor this and ensure that appropriate breaks are taken. Drivers and their employers are legally required to accurately record their activities, retain the records and produce them on demand to transport authorities who are in charge of enforcing regulations governing drivers’ hours of service.