Speed limits frame expectations for drivers and other roadway users. Properly set speed limits provide a safe, consistent, and 

reasonable speed to protect drivers, pedestrians, and bicyclists along the roadway. At the same time, speed limits can be a source 

of frustration and confusion; for example, not all drivers like to travel at the same speed, and some people may not understand 

why the speed limit changes on a particular road. In addition, community residents often have concerns that traffic is moving too 

fast through their neighborhoods. Understanding the engineering principles and processes used to set speed limits and learning the 

terminology used to describe them are the first steps in reducing drivers’ frustration or confusion and encouraging compliance. 

TYPES OF SPEED LIMITS

STATUTORY SPEED LIMITS

Statutory speed limits are established by State legislatures for 

specific types of roads (e.g., Interstates, rural highways, urban 

streets) and can vary from State to State. They are enforceable by 

law and are applicable even if the speed limit sign is not posted. 

Examples of statutory speed limits include:

 25 mph in residential or school districts,

 55 mph on rural highways, and 

 70 mph on rural Interstate highways. 

POSTED SPEED LIMIT

Posted speed limits (sometimes called regulatory speed limits) are 

those that are sign-posted along the road and are enforceable by 

law. A posted speed limit could be the same as the statutory speed 

set by the State legislature, or it could be established by a city, 

county, or State transportation agency as an adjustment to the 

statutory speed limit. Some cities and counties will establish 

a blanket speed limit for roads in their jurisdictions. Those limits 

are generally posted at the city limits or county lines. The posted 

speed limit can differ from the statutory speed limit; in these cases, 

the posted speed limit is determined using an engineering speed 

study and takes priority over the established statutory speed limit.

“SPECIAL CONDITIONS” SPEED LIMITS

School zone speed limits are used in specific locations during the 

hours when children are going to and from school. Most States use 

a school zone speed limit of 15 to 25 mph in urban and suburban 

areas. 

Work zone speed limits are set as part of the work zone’s traffic 

control plan, which is used to help facilitate safe and efficient 

movement of traffic through a work zone. Factors that influence 

work zone speed limits can include:

 The posted speed limit when the work zone is not present,

 The location of the work zone and workers in relation to traffic,

 The type of traffic control (e.g., cones, barrels, concrete 

barriers), and

 The complexity of the work zone (e.g., lane shifts, narrowed lanes).

Variable speed limits are displayed on changeable message signs 

(CMS) at locations where roadway conditions regularly require 

speeds to reduce more than 10 mph below the posted speed 

limit. These instances typically occur due to weather conditions, 

congestion, traffic incidents, and/or work zones.

Advisory speeds are a non-regulatory speed posted for a small 

portion or isolated section of a roadway (e.g., a sharp curve, an exit 

ramp) to inform a driver of a safe driving speed. They are set using 

an engineering speed study and in accordance with guidance in 

the Manual on Uniform Traffic Control Devices (MUTCD).

Basics

STATUTORY POSTED SCHOOL ZONE WORKZONE VARIABLE ADVISORY

SCHOOL

7:30-8:30 AM

2:30-3:30 PM

WORK

ZONE

SPEED LIMIT
http://safety.fhwa.dot.gov

ESTABLISHING SPEED LIMITS

State and local transportation agencies recommend and set 

appropriate speed limits by completing engineering speed 

studies and following the guidance presented in the MUTCD. 

Practitioners may also use a supporting web-based tool called 

USLIMITS2, which provides an objective second opinion and 

helps support speed-limit-setting decisions. At times, agencies 

may need to implement speed management countermeasures to 

achieve the desired speed for a particular roadway (e.g., in areas 

with high pedestrian and bicyclist activity). Review FHWA’s Speed 

Management Countermeasures: More than Just Speed Humps for 

more information on speed management countermeasures.

SPEED CONCEPTS

Besides the types of speed limits, there are concepts relating to 

speed that are important to understand. 

 Before a new road is built, engineers choose a design speed in 

order to guide their design decisions and prepare the plans.

 Once the road is built, engineers will evaluate the existing 

speeds by measuring the operating speed. They often do 

this by measuring the speed that 85 percent of drivers are 

travelling at or below, called 85th percentile speed.

 Used extensively in the traffic engineering field, the 85th 

percentile speed is based on the premise that the majority of 

drivers choose reasonable speeds for given road conditions 

and should be accommodated. 

CORRECTING COMMON MISCONCEPTIONS

 The Federal Government does NOT set or enforce speed limits; 

this authority belongs to the State and local agencies that have 

jurisdiction over the road.

 The 85th percentile speed is not the only factor practitioners 

evaluate when determining an appropriate speed limit; 

they complete engineering speed studies and often utilize 

supporting tools like USLIMITS2.

 Simply lowering the speed limit does not guarantee motorists 

will drive slower; speed management countermeasures may 

have to be implemented along the roadway.

SPEED SAFETY FACTS

 Drivers who exceed the posted speed limit or drive too fast for 

conditions are involved in nearly one-third of all fatal crashes.

 Only 13 percent of speeding-related fatalities occur on 

interstate highways.

 More than 40 percent of speeding drivers in fatal crashes were 

considered to be alcohol-impaired.

What is included in an engineering speed study? 

 Speeds of motorists in normal conditions

 Traffic volume

 Roadway type (e.g., interstate, freeway, 

city street)

 Roadway features (e.g., curves, hills, 

number of lanes)

 Roadway setting (e.g., urban, rural, 

residential, woodland, farmland)

 Number and spacing of driveways or 

intersections

 Sight distances

 Presence of on-street parking

 Pedestrian or bicyclist activity

 Crash history

 Pavement condition

23 mph 42 mph 58 mph

90%

50%

10%

Average risk of death for a pedestrian

at impact rises as speed increases 

SOURCE: AAA Foundation for Traffic Safety, Impact Speed and a Pedestrian’s Risk of Severe Injury or Death, September 2011.

FHWA-SA-16-076

FOR MORE INFORMATION

To learn more about speed 

management, visit FHWA’s 

Speed Management Safety 

web page.

For more information on speed 

management, check out FHWA’s Speed 

Management Countermeasures: More than 

Just Speed Humps fact sheet.