Websites by the research team found that all 31 States use the operating speed approach. More than half of the States reviewed consider roadside development or land use, traffic condition or volume, sight distances, the maximum or minimum speed allowed in the State, as additional factors, while at least three States also consider alignment or crosssectional factors such as horizontal/vertical curves, lane width, and pavement width. 2. Road Risk : This approach is primarily used in Canada and New Zealand and determines the speed limit based on the risks associated with the physical design of the road and the expected traffic conditions. Essentially, the speed limits are set according to the function or classification of the roads, and then adjustments to the speed limit are made based on the relative risk introduced by road and roadside design features. Since this method may result in speed limits well below the 85th percentile speeds, additional enforcement may be needed if countermeasures to reduce operating speeds are not employed. 3. Expert System : This approach recommends a speed limit based on a computer algorithm that uses knowledge and inference procedures that simulate the judgment and behavior derived from knowledgeable people, i.e., the “experts.” The first expert system approach was developed in the late 1980s by Australia, and currently USLIMITS2, a Web-based tool developed by FHWA, is used on a limited basis as an expert system in the United States. 4. Injury Minimization/Safe System : This approach sets speed limits based on collision types that are likely to occur, the resulting impact forces, and the human body’s tolerance to withstand those forces. This approach is based on the scientific link between speed limits and serious crash prevention, placing a high priority on road safety. It is used in Australia, the Netherlands, and Sweden but is not commonly used in the United States. This may be due to lack of exposure to this method, or because the resulting low speed limits are not viewed as appropriate by some jurisdictions 5. Citywide of Default : This approach sets speed limits by government actions. The report discusses city efforts to set uniform speed limits for safety, such as a 20 mph speed limit on residential streets. States generally control statutory speed limits, and several U.S. cities have recently campaigned to gain the authority from their state legislatures to set lower citywide default speed limits. For example, Boston, Massachusetts; New York City, New York; and Seattle, Washington, now have the ability to set a 25 mph speed limit citywide. Portland, Oregon, has the authority to set residential streets at 20 mph. Other countries are also implementing citywide speed limits.
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