Why Speed Limits?Speed limits are supposed to do two things.The primary purpose of speed limits is toenhance safety by reducing risks imposedby drivers speed choices. The intent is toreduce disparities in speeds and reduce thepotential for vehicle conflicts. A relatedfunction of speed limits is to provide thebasis for enforcement and sanctions forthose who drive at speeds excessive forconditions and endanger others.

LAW

Prima facie speed limits are those which, “onthe face of it,” are reasonable and prudentunder normal conditions. Normally a drivermay exceed a prima facie limit if it is safe todo so; however it is up to the driver to provethat he was driving in a safe manner underexisting conditions.

In Colorado, basic prima facie speed limits are:� 20 mph on narrow, winding mountain roads� 25 mph in any business district� 30 mph in any residential district� 40 mph on open mountain highways

Absolute speed limits are those which maynot be legally exceeded under any circum-stances. These are:� 65 mph on open highways� 75 mph on rural interstate routes

Section 42-4-1102, Colorado Revised Statutes,requires that speed limits are not to be higher orlower than the basic prima facie speed limitsunless a Traffic Investigation has justified thechange. This law applies to all State Highways,County Roads and City Streets. For State High-ways, including portions within municipal corpo-rate limits, the Investigation to justify an increaseor decrease of existing speed limits is normallyconducted by, and approved by, the Safetyand Traffic Engineering Branch of the ColoradoDepartment of Transportation.

Each Investigation to determine an appro-priate speed limit should consider the follow-ing factors applicable to the portion of roadbeing studied.

� Prevailing speed data (85th percentile)� Roadside development� Accident experience� Road characteristics� Pace speed� Parking practices/pedestrian activity

The use of vehicle speed data as one of thefactors evaluated for selecting a suitablespeed limit is based upon the followingfundamental concepts deeply rooted in theUnited States system of government andlaw:

A) Laws cannot be effectively enforcedwithout the consent and voluntarycompliance of the public;

B) Laws are established for the protectionof the public and the regulation of theunreasonable behavior of a few indi-viduals;

C) The normally careful and competentactions of a reasonable person shouldbe considered legal;

D) Most drivers are reasonable people whowill drive carefully at a speed which issuitable for existing conditions.

MISCONCEPTIONS

It is a popular misconception that reducingthe speed limit will automatically slow thespeed of traffic, while raising the speed limitwill automatically cause an increase in thespeed of traffic.

“Before and After” speed studies show thatthere are no significant changes in vehicle

speeds after speed limits are changed.“Before and After” accident studies usuallydo not show any significant change inaccident rates after speed limits are in-creased or decreased. National studies gofurther and say that “it is generallyat the upper boundary of a speed rangewhere crash involvement rates are lowest.”

UNREALISTIC SPEED LIMITS

Traffic investigations have shown that mostpeople will drive the roadway as theyperceive the conditions and will ignore aspeed limit that is unrealistically too low ortoo high. A realistic speed limit is voluntarilyobeyed by the reasonable majority andmore enforcement effort can be applied tothe unreasonable few who drive too fast ortoo slow.

An unrealistic speed limit that is “too low”will:

A) Make the behavior of the majorityunlawful;

B) If enforced - cause antagonism towardenforcement personnel and traffic lawsin general;

C) Create a bad image of the communityfor visitors and tourists;

D) Result in speed differentials in the trafficflow.

Establishing

Realistic

SPEED

LIMITS

Speed as it relates to accident causality isprimarily related to speed differentials.Speed differential is the range of vehiclespeeds within the traffic stream. A largevariation in these speeds complicates thedriving task and necessitates sudden brak-ing, multiple lane changes and other com-pensating driving maneuvers.

A speed limit properly set, will establish amiddle ground for all drivers encouragingsome to speed up while enticing others toslow down. This then reduces turbulencewithin the traffic stream and limits conflictpoints and reduces accidents.

There is no question, however, that speedplays a role in accident severity. Once anaccident has begun to occur the degree ofdamage to a vehicle and its occupants isdirectly related to the speed the vehicle isgoing.

REALISTIC SPEED LIMITS

An appropriate, “just right” speed limit willresult in the maximum number of vehiclestraveling at about the same speed, thusreducing conflicts caused by speed differ-entials. The 85th percentile speed, thatspeed at or below which 85% of the traffic ismoving, is widely accepted as being closestto that “just right” speed limit - a case ofMajority Rule. Of course, other Traffic Investi-gation factors must be taken into consider-ation.

Following are some reasons for establishingrealistic speed limits:

A) To provide guidance to the driver,especially strangers to the area, as towhat is a suitable speed for normalconditions;

B) Reasonable speed limits with adequatesigning tend to reduce the speed differ-ence between vehicles. The accidentrate is less when the majority of vehiclesare traveling at about the same speed;

C) To furnish enforcement personnel with aguide as to what is an appropriatespeed for a segment of road so thatenforcement actions may be consistentand fair;

D) To improve the overall credibility of alltraffic control devices.

WHAT YOU CAN DO

Anyone may report a road segment wherethe speed limit seems to be too high or toolow. If the segment is a portion of countyroad or city street, contact should be madewith that county or municipality. If the seg-ment is a portion of the State HighwaySystem, including U.S. and Interstate routes,contact should be with the Region’s Trafficand Safety Engineer of the Colorado De-partment of Transportation.

Safety and Traffic Engineering Branch

4201 East Arkansas Avenue

EP Suite 770

Denver, Colorado 80222

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