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A method is presented to determine variable speed limits, taking account of road geometry and vehicle characteristics. This method is based on a mathematical vehicle model and a detailed digital map that contains every piece of data that may be relevant from a safety point of view. Objective safety criteria such as tyre-road friction and sight distance are used. The main purpose of calculating new limits is to improve safety and to increase the acceptance of these limits by drivers. These safe speed limits have been integrated into the Spanish intelligent speed adaptation system and a new interface has been developed in order to alert the driver before they reach dangerous stretches of road. Furthermore, this system has been tested under real driving conditions. Test results show that speeding on sharp bends is reduced and traffic is more homogeneous. Both effects lead to greater traffic safety.