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Home > Technical Subjects >Drainage and Erosion Control >Laws Affecting Roads Around Waterbodies

How are Speed Limits Set?

Watch the 11 minute video entitled “Establishing Speed Limits in Maine”

When establishing a speed limit, the main premise is that most drivers are prudent and will voluntarily comply with a reasonable speed limit. To determine what is reasonable, engineers measure drivers' speed on a section of roadway, the speed at which 85% of drivers are at or below is the standard for determining a speed limit. A properly set speed limit will be within 3 miles per hour (±) of this observed speed. The posted speed limit will then be rounded to the nearest 5 miles per hour.

This is a graphic showing the 85th percentileResearch has shown that the 85th percentile speed is the speed where accident involvement is the lowest. Reducing the speed limit below what is warranted can actually be detrimental to safety.

Measurements to determine the 85th percentile value are made under free flowing and ideal traffic conditions. This means that if speeds are measured on any section of road, 85% of the motorists will be driving at or below the 85th percentile speed.

Speed zoning is based upon several fundamental concepts deeply rooted in our American system of government and law:

  • Driving behavior is an extension of social attitude, and the majority of drivers respond in a safe and reasonable manner as demonstrated by their consistently favorable driving records.

  • The normally careful and competent actions of a reasonable person should be considered legal.
  • Laws are established for the protection of the public and the regulation of unreasonable behavior on the part of individuals.
  • Laws cannot be effectively enforced without the consent and voluntary compliance of the public majority.

In Maine, State law (29-A § 2073 §-1 and 2075, §-3) authorizes the MaineDOT, with the approval of the Chief of the State Police, as the only legal entity to create or change a speed limit on a public way which includes,State and State Aid Highways and townways. On September 21, 2001, the law changed to allow certain “qualifying municipalities” to have the full responsibility and authority for setting speed limits on local roads….. if they choose that option. A “qualifying municipality” is one that (1) has a population over 2,500 as measured by the last US Census, or (2) employs a Professional Engineer (PE) licensed in Maine. Qualifying roads are ONLY townways which are federally classified as “local” roads. 

If a town creates or changes a speed limit or simply erects speed limit signs without going through the proper process, there is no legal authority to the change and it is unenforceable. 

Except when conditions or other regulations require a lower speed, the following are maximum rates of speed, especially if signs are not posted:

  • 15 m.p.h. in a school zone at specific times of the day,
  • 25 m.p.h. in a business or residential area or built up portion, unless otherwise posted.
  • 45 m.p.h. on all other public ways, unless otherwise posted.
  • However, the noted 25 and 45 mph limits are usually unenforceable if they do not have MaineDot approval

A “school zone” was also redefined in 2001 through LD 843 which became Public Law 2001, Chapter 145 and became effective on May 14, 2001. A “School zone” “means the portion of the public highway abutting improved school property or 300 feet on either side of a school entrance, whichever is greater.”

" Improved school property” now is defined as “the developed portion of school property including driveways, parking lots, playgrounds, athletic fields or school buildings.”

If there are roads in your town under MaineDOT jurisdiction and you feel the need to create or change a speed limit, a municipal official must request the change in writing to your local MaineDOT Regional Traffic Engineer. A field study will be made and then a recommended speed will be forwarded to the MaineDOT Commissioner and the State Police. Then the town will be notified of the speed limit and be responsible for erecting the standard and minimum 24" x 30" black-on-white signs in the proper locations if the change is on a town way. If the change is on a State road, then the MaineDOT will make the signing changes.

 List of Regional Traffic Engineers:


MaineDOT Region
Traffic Engineer
Telephone Number
Region 1
Tim Soucie
Region 2
Dave Allen 
Region 3
Dennis Emidy
Region 4
Bruce Mattson
Region 5
Presque Isle
Jim McGinnis


During the field study, there are several factors that engineers use to determine an acceptable speed limit. In fact, if your town is a “qualifying town” and chooses to set local speed limits, local officials should be studying the same factors.

  • geometric design of the road,
  • public and private access points,
  • the number of intersections,
  • the number of roadside businesses,
  • observed travel speeds of traffic,
  • the 85th percentile of the observed speed ranges,
  • total accidents in a 3 year time frame,
  • accidents just from driveways and intersections within a 3 year period,
  • a series of test runs on that section of road driving a certain speed evaluating safety and drivability.

There are two types of speed limits: one is “regulatory” and the other is “advisory”. A regulatory speed limit is set by MaineDOT and printed black on a white background. The minimum size of regulatory speed limit signs is 24 X 30 inches. Also, regulatory speed limit signs shall be in increments of 5 M.P.H. A special regulatory speed sign that drivers need to be aware of is the school speed limit in school zones. The posted speed is in effect when school is in session before school begins, after school, and at recess.

The other type of speed limit is an advisory limit. This black on yellow speed limit sign is used to advise motorists of a comfortable speed at which to travel when different situations lie ahead. It is used with a warning sign like a right or left curve sign. The standard size for these signs is 18 X 18 inches, except in cases where it is 24 X 24 inches because it supplements a 36 inch and larger warning sign. Another type of advisory speed limit sign can be found in work zones. These signs are black on orange. These are used to advise drivers of construction ahead and provide work crews safety.



This page last updated on 8/20/14