At the request of safety advocates and the city of Boston, and working with other senators, I added language to the Senate version of the recent municipal modernization bill that will give municipalities more flexibility to set lower speed limits.

Currently, under state law, vehicles may not be driven above speeds that are “reasonable and proper”, depending on conditions. The maximum “reasonable and proper” speed under ideal conditions in thickly settled areas is 30 miles per hour. All of my district is “thickly settled” — having houses closer together than 200 feet. Municipalities can set lower limits in school zones, but do not have general flexibility to set lower limits.

To set a lower limit (or higher limit) under current law, a municipality must go through an arduous procedure to establish the “85th percentile speed” based on actual speed studies. In other words, the town must do expensive traffic monitoring to compute the speed at or below which 85% of the vehicles are actually operating on the particular road segment. On some roads, this creates real risk that an effort to lower speed limits could end up increasing speed limits. It is very consistent with the spirit of the municipal modernization bill to make it easier for municipalities to lower limits without so much red tape.

The new language would give municipal authorities — the selectmen, or in a city, the traffic director — authority to set 25 mile per hour limits in thickly settled zones without further approval. They would also be able to lower the limit to 20 miles per hour in selected safety zones. The new powers would take effect only upon a vote of the municipality’s legislative body — the town meeting or town/city council.

From a safety standpoint, there is no question that lower speeds are desirable. In congested areas, the probability of collision goes up with higher speeds. Also, the severity of injuries from collisions goes up dramatically with speed.

The practical question is: how can we actually change the speed of vehicles? All of my experience as a driver and as someone involved in setting speed limits is that people drive at the speed they feel comfortable. That varies according to mood and road conditions and, of great importance, according to road design. Long straightaways with wide lanes, especially with green lights visible ahead, induce drivers to accelerate to unreasonable speeds. This is spectacularly exemplified by Beacon Street in Boston where cars routinely exceed 50 miles an hour in a very dense area.

I believe that, in areas where motorists tend to drive too fast, the solution is traffic calming road redesign. If motorists are driving 50 in a 30 mile per hour zone, they will likely continue to drive 50 in a 25 mile per hour zone.

I also share reservations that if local authorities can set lower speed limits with no oversight, they will create speed traps and/or set unreasonably low speeds in response to campaigns by well-organized neighborhoods which will divert traffic into neighborhoods who are less well-organized. I have thought of the state department of transportation as the guardian of fair uniformity in the setting of speed limits.

All of that said, road redesign is slow and expensive, and safety is an urgent priority. I was ultimately persuaded by MassDOT’s decision to endorse the language and was pleased to push for its adoption.

Through the efforts of Representatives Hecht, Livingstone, Provost and others, the language was also included in the House draft and is therefore likely to become law. Local authorities will have to make the ultimate choices within the new more flexible rules.