A tachograph is a small circle of paper that is inscribed with a line as a truck drives. When the truck is moving, the line is drawn. When the truck stops, the line stops being drawn, but the disk continues to rotate. These devices have been included in European trucks by law since the 1980s, but can be tampered with. Digital tachographs have been developed to minimize this problem.
By the line drawn, a tachograph indicates for how long and how fast a truck drove. The lack of marks for when the truck was idle are proof that a delivery was dropped off. If the truck is in an accident, the mark for that will show up as well. Using a microscope, the line made by the pen can be examined for jumps made when the truck was in an accident.
Tachographs can be tampered with. Drivers can block the pen from writing even when the truck is moving or mark a line to show that they drove when the truck has stopped. Another method that doesn't require forgery would be if the driver simply "forgot" to insert the paper disk inside the tachograph before leaving on a trip. A second tachograph can even be rigged to the same car so that another driver has proof of a drive that they didn't make.
Digital tachographs have been developed, which are less likely to be tampered with than the ones using paper disks. These tachographs display the time and distance the truck has gone digitally. The information gathered during the drive is stored on smart cards, which are turned in as proof of work. A digital tachograph's information will often be encrypted to deter hackers.
Tachographs have many uses besides serving as work logs. In Germany, a tachograph can be used as a record to punish speeding. In places where drivers' breaks are mandatory, a tachograph proves that they are being taken and that a driver isn't working even when exhausted. The disks can also be used as proof in claims for unpaid work or when an employer has pushed its employees to work beyond unreasonable limits.
A tachograph can also serve as a valuable data mine for a company. Compiling data from all tachographs can show a great deal about how often company policies are followed. They can also show work patterns and how well workers were able to meet goals.