Personal Track Safety (PTS) is a system of safer working practices employed within the United Kingdom designed to ensure the safety of railway workers who have to work on or near the line. The principal hazards include collisions between a rail vehicle and a track worker, electrocution from traction power sources (third rail, fourth rail, OHLE) and trips and falls. The last could compound the other two (e.g. a worker could fall onto an electrified third rail). PTS ensures that rail workers are aware of their surroundings so that they do not enter situations where the aforementioned accidents are likely to occur, are able to move around the lineside safely and are able to react appropriately to circumstances (e.g. the approach of a train). Compared to road vehicles, trains have a much greater stopping distance at the same speed, but often travel much faster than road vehicles. Unlike road vehicles, they cannot swerve out of the way of obstructions. Trains cannot be relied upon to stop for rail workers. Hence it is the duty of the track worker to remain in or retire to a safe location on the approach of a train. It is important that a lookout is kept (often working as a team). In order that trains can indicate their presence to workers, orange high visibility clothing must be worn. Clothing that is yellow, green or red is disallowed because those colours are the colours of signal flags. Sentinel Card A Sentinel Card is required before anybody is allowed to work within the boundary of Network Rail tracks in the UK (on or near the line). It is also a requirement in the Republic of Ireland. Any potential employee must undergo a medical and a drug and alcohol test before attending a "personal track safety" course. The Sentinel Card is a smartcard which links to an online database, giving details of the holder's competencies and other details – this may include: Medical certificate (expiry dependent on age) PTS (AC or DCCR) (expiry 2–5 years) PICOP, COSS, IWA or others. Deregulation in the national railway service in the UK has meant that increasing numbers of subcontractors are being employed within the track environment. Contractors are often used for specific limited projects and are often from companies that have not previously been considered as having "railway pedigrees or culture". To work on or near the track involves certification for