Speed limit is one of the traffic problems in developing countries due to the influence of speed on roadway safety. While most drivers are assumed to be capable of making reasonable judgments about appropriate driving speeds, speed limits are necessary because of the significant risks drivers can impose on others. The actual speed on these roads is often higher than the legal limit indicating that many road users do not consider the speed limits seriously. The differentiated speed limits allowing for higher or lower speed limits could be a way to improve acceptance of the speed limits. Most traffic engineers believe that speed limits should be posted to reflect the maximum speed considered to be safe and reasonable by the majority of drivers using the roadway under favourable conditions. The aim of this study was to determine the impact of speed limit increases on mobility and the environment in urban areas. The results show that a large number of road users’ drive above limits set by national or local authorities. Reduced speed is likely to bring about the reduction in average travel speed and have a positive impact on the number of accidents and accident severity. The other benefits are also derived including reduced the fuel consumption and the vehicle operating cost, and significant reductions in environmental impact like vehicle emissions. The results show that the effects of a 5 km per hour reduction in travel speed are greater at lowers speeds and reductions in travel speeds over 50 km per hour bring dramatic decreases in crash risk. Reduction in vehicle speed will be followed by the reduction in fuel consumption and vehicle operating cost of vehicle. Besides the cost of increasing fuel, there are a number of other vehicle operating costs such as tire wear, lubricating oil, and spare parts that tend to increase with increasing speed. The relationship between changes in speed limits and vehicle emission, when the speed limit was lowered there was reduction in nitrogen oxides (NOx) emission and carbon monoxide (CO) emission.