The design speed is a tool used to determine geometric features of a new

road A road is a thoroughfare used primarily for movement of traffic. Roads differ from streets, whose primary use is local access. They also differ from stroads, which combine the features of streets and roads. Most modern roads are paved. Th ...

street A street is a public thoroughfare in a city, town or village, typically lined with Building, buildings on one or both sides. Streets often include pavements (sidewalks), pedestrian crossings, and sometimes amenities like Street light, streetligh ...

during road design. Contrary to the word's implication, the design speed of the road or street is not necessarily its

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

speed limit Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed, express ...

or maximum safe speed; that can be higher or lower. Choosing a design speed means finding a balance between several interests which compete for priority, such as high vehicle speeds to allow drivers to

travel Travel is the movement of people between distant geographical Location (geography), locations. Travel can be done by Pedestrian, foot, bicycle, automobile, train, boat, bus, airplane, ship or other means, with or without Baggage, luggage, a ...

to their destinations quickly versus low vehicle speeds for the

safety Safety is the state of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk. Meanings The word 'safety' entered the English language in the 1 ...

of people outside the vehicle (such as

pedestrian A pedestrian is a person traveling on foot, by wheelchair or with other mobility aids. Streets and roads often have a designated footpath for pedestrian traffic, called the '' sidewalk'' in North American English, the ''pavement'' in British En ...

s and cyclists), or quick movement of peak traffic ( traffic engineering) versus maximising the economic development potential of the street (

urban planning Urban planning (also called city planning in some contexts) is the process of developing and designing land use and the built environment, including air, water, and the infrastructure passing into and out of urban areas, such as transportatio ...

Geometric features

The design speed chosen for a high-speed roadway is a major factor in choosing superelevation rates and radii of

curves A curve is a geometrical object in mathematics. Curve(s) may also refer to: Arts, entertainment, and media Music * Curve (band), an English alternative rock music group * Curve (album), ''Curve'' (album), a 2012 album by Our Lady Peace * Curve ( ...

, sight distance, and the lengths of crest and sag vertical curves. Roads with higher design speeds require wider lanes, sweeping curves, wider recovery areas, broader clear zones, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Roads and streets with lower design speeds require narrower lanes, sharper/tighter curves, smaller or no clear zones, less banking, less sight distance, and sharper hill crests and valleys.

Speed limit variance from design speed

While a road's design speed is sometimes used to determine an initial speed limit, it is an imperfect measure of the maximum

speed In kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. Intro ...

at which a

motor vehicle A motor vehicle, also known as a motorized vehicle, automotive vehicle, automobile, or road vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on railway track, rails (such as trains or trams), does not fly (such ...

can be operated for reasons including: * It is only a theoretical or laboratory measurement created before a road is even built. * Design speed is not necessarily road's maximum safe speed. As highway design incorporates a significant factor of safety, drivers can travel faster than design speed without difficulty when good weather conditions are present. * The highest design speed for a road or segment is the design speed of its least favorable part. For example, given a road segment with a design speed except for a curve with a design speed, the entire segment would have a design speed. The road may have a advisory speed on the curve and higher safe operating speeds elsewhere. * The design speed may be higher than legislated

caps, so it would not be legal to sign some roads at their design speeds. * It is based on the capabilities of vehicles and roadways that existed at or before the design speed was determined. Vehicular and roadway technologies generally improve over time. Therefore, as time elapses from when a roadway's original design speed was determined, it is increasingly likely that a design speed will underestimate the maximum safe speed. Recognizing the limitations on the use of the design speed for speed limit determination, "operating speeds and even posted speed limits can be higher than design speeds without necessarily compromising safety" and "arbitrarily setting lower speed limits at point locations due to a lower inferred design speed is neither effective nor good engineering practice."

Evolution

The concept of design speed is evolving. The definition in the 1994 edition of the

AASHTO The American Association of State Highway and Transportation Officials (AASHTO) is a standards setting body which publishes specifications, test quality control, protocols, and guidelines that are used in highway design and construction through ...

''Green Book,'' was "the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway." Most US states used this definition. In 2004, the first sentence was changed to "a selected speed used to determine the various geometric design features of a roadway." This reflects the fact that meeting a minimum design speed is not enough to ensure a safe roadway. Recently, the concept of design consistency has been used instead of minimum design speeds. This attempts to connect driver's expectations about the roadway with the roadway design. It uses driver behavior models to predict vehicle speeds on highway segments, and compares the predicted speed on adjacent segments. Significant reductions in speed from one segment to the next are flagged as locations where drivers may end up driving too fast for road conditions. A major shift in philosophy is also taking place regarding design speed of urban and suburban streets. Highway engineers would measure the prevailing speed on a road, round up to the next multiple of , and design the road for that speed, assuming that it would be safe. Recent research and design practices have focused on using the street design to influence drivers to choose a speed appropriate for the neighborhood. This speed has been called the "target speed," and is ideally equal to the speed limit.

Factors

When roads are planned, the selected design speed may be based on or influence several factors, including: * geometric design of road features * planned operating speed * legislated speed limit caps * anticipated traffic volume * the road's functional classification

References

Bibliography