IMOLA, Italy - The deaths of drivers Ayrton Senna and Roland Ratzenberger at the 1994 San Marino GP were the last in Formula 1 and long may that be so.This is largely to the safety review prompted by their crashes and ongoing measures to reduce the almost annual incidence of such tragedies.Here's a review of the main safety measures enacted in F1 since then:HEAD AND NECK SUPPORTSenna died due to head injuries and as a result protection of the head and neck is the principal safety priority for drivers, as it is trauma to those regions that are most likely to cause death in the short time before medical personnel can attend to an injured driver.F1 helmets are subject to testing to ensure they can withstand a heavy impact. Modern helmets are fire resistant and made of carbon-fibre, Kevlar and polyethylene. The visor has removable strips which can be torn off if debris or liquid obscures a drivers vision. In 2003, the FIA made Head and Neck Support (HANS) device compulsory.The device, attached to the rear of the helmet, is connected to the interior of the cockpit adjacent to the safety-belt mounting. It prevents rapid and excessive head movement during crashes.MAKING F1 CARS SAFERF1 cars are safer than they have ever been, able to withstand powerful impacts. Robert Kubica's terrifying crash at the 2007 Canadian GP was evidence of just how much the cars can withstand. Kubica sustained minor injuries from a collision into a wall that was heavier in impact than Senna's at Imola. Focus has been placed on the "survival cell" - the area in front of the engine and surrounding the driver. Comprised of carbon fibre, the cell is designed to resist impacts and penetration by debris during crashes. The opening of the cockpit has been increased in recent years to prevent drivers being trapped inside during fires and enable safer and easier removal of injured drivers. Crash tests are more rigorous as they need to withstand impacts from all directions. Tethers prevent wheels detaching during crashes to protect other cars, marshalls and spectators. Another key change has been the switch from metal fuel tanks to rubber-coated, carbon-fibre tanks which are much less likely to rupture in a crash.Professor Sid Watkins was the doctor on duty at the 1994 San Marino GP and performed an emergency tracheotomy on Senna. It was a particularly tragic moment for Watkins, as he was a close friend of Senna and he had done much to improve the series' safety in the 1970s. It was at his insistence that many of the post-Senna medical improvements were made. Medical cars, with paramedics on board, are stationed at the circuit and the FIA claim they can can reach the site of any crash within 30 seconds. The trackside medical center has a surgeon and other staff on site and there are helicopters at every race to transport seriously injured drivers and personnel to nearby hospitals. The FIA reports that there are an average of 130 medical staff at each race.TRACK DESIGNGone are the days of straw bales and, for the most part, concrete walls on the edge of tracks. Modern circuit design puts a premium on safety. With the exception of Singapore's Marina Bay, which has the inherent restrictions of all street circuits, new tracks designed since the 1990s have generous run-off areas at high-speed corners, unlike the Tamburello bend at Imola where Senna lost his life. The efficacy of trackside barriers has been greatly increased and they are able to absorb most of the energy of a high-speed crash. Another safety feature is the introduction of the safety car to slow the field while the scene of a crash is cleared. Pits-lane speed limits have also been introduced and progressively lowered to prevent racing in an area full of crew and trackside personnel.SLOW PROGRESSThe relative safety of F1 compared to the past has evolved slowly. It is staggering to the modern fan to realize how lax safety standards were in the past. Helmets became compulsory in 1953, fireproof overalls were first introduced in 1963 and seatbelts were implemented in 1972. Crash tests were introduced only in 1985. Three-time World Champion Jackie Stewart was a strong advocate in the 1970s for improving safety standards, and his work, along with that of Sid Watkins, and the tragedies of Imola 1994 have vastly improved the series.