Chapter 90, Section 18 authorizes the posting of numerical speed limits on all roadways in Massachusetts. The foundation for the actual posting of a speed limit is a thorough traffic engineering study. After a study has been completed, a Special Speed Regulation is drafted and approved by the governing authority of the roadway, the Registry of Motor Vehicles, and MassHighway. All posted regulatory speed limit signs must adhere to this approval process. If a speed limit is posted without this procedure, it is in violation of Chapter 90, Section 18, and is therefore considered illegal and unenforceable.
A prerequisite to establishing speed regulations and posting speed limits is a comprehensive engineering study at each location where speed control is contemplated. The purpose of the study is to establish a speed limit that is safe, reasonable, and self-enforcing. The most important step is measuring the prevailing speeds of motorists on a particular section of roadway under ideal conditions. The speed at or below which 85 percent of the motorists travel is the principle value used for establishing speed limits. This is commonly referred to as the 85th percentile speed. This method is based on numerous studies which indicate that the majority of motorists are prudent and capable of selecting safe speeds; therefore, speeds established in this manner meet the legal requirement that they be “reasonable and proper.” The 85th percentile speed is the national standard for establishing safe speed limits.
In Massachusetts, numerical limits are based on ideal conditions. More specifically, the posted speed limits represent the maximum safe speed under ideal driving conditions. It is the responsibility of each motorist to reduce his/her speed for unfavorable weather conditions, for poor visibility, for heavy traffic volume, for substandard vehicle conditions, and for his/her own driving deficiencies. Posted speed limits also serve as an invaluable guide to enforcement officers as to what is a reasonable maximum speed for ideal conditions.