Speed Limits - How Are They Set?

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.

In Maine, State law authorizes the MDOT, with the approval of the Chief of the State Police, as the only legal entity to create or change a speed limit. If towns create or change a speed limit or simply erect speed limit signs, there is no legal authority to the change and it is unenforceable.

If speed limits are not posted on a public way, then the following "defaults" are in effect:

15 m.p.h. in a school zone during recess or closing hours,

25 m.p.h. in a business or residential area or built up portion,

45 m.p.h. on all other public ways

Therefore, if your town feels the need to create or change a speed limit, you must request the change in writing to the MDOT Traffic Engineer or your local MDOT Division Traffic Engineer. A field study will be conducted, a recommended speed will be forwarded to the DOT Commissioner and the State Police, and the town will be notified of the decision. The town is responsible for erecting the standard black-on-white signs in the proper location, when a change is located on a townway; if the change is on a state road, then the MDOT is responsible for placement of the sign.

During the field study, there are several factors that engineers use to determine an acceptable speed limit.

geometric design of the road
public and private access points
the number of intersections
the number of roadside businesses
observed speeds of traffic
the 85th percentile of the observed speed ranges
the 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 while driving a certain speed evaluating safety and derivability.

There are two types of speed limits: one is "regulatory" and the other is "advisory". The standard size of regulatory speed limit signs is 24x30 inches, and 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 a 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 motorist 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 18x18 inches, except in cases where it is 24x24 inches because it supplements a 36 inch or larger warning sign. Another type of advisory speed limit sign can be found in work zones. These signs are black on orange, and are used to advise drivers of construction ahead and provide work crews safety.

Whether a speed limit is regulatory or advisory, it is used to inform motorist about appropriate suggested traveling speeds.