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

road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types o ...

street A street is a public thoroughfare in a built environment. It is a public parcel of land adjoining buildings in an urban context, on which people may freely assemble, interact, and move about. A street can be as simple as a level patch of di ...

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

vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...

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 - expre ...

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 high vehicle speeds to allow drivers to

travel Travel is the movement of people between distant geographical locations. Travel can be done by foot, bicycle, automobile, train, boat, bus, airplane, ship or other means, with or without luggage, and can be one way or round trip. Travel c ...

to their destinations quickly versus low vehicle speeds for the

safety Safety is the state of being "safe", the condition 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 There are two slightly di ...

of people outside the vehicle (such as

pedestrian A pedestrian is a person traveling on foot, whether walking or running. In modern times, the term usually refers to someone walking on a road or pavement, but this was not the case historically. The meaning of pedestrian is displayed with ...

s and

cyclist Cycling, also, when on a two-wheeled bicycle, called bicycling or biking, is the use of cycles for transport, recreation, exercise or sport. People engaged in cycling are referred to as "cyclists", "bicyclists", or "bikers". Apart from two ...

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

urban planning Urban planning, also known as town planning, city planning, regional planning, or rural planning, is a technical and political process that is focused on the development and design of land use and the built environment, including air, water, ...

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), a 2012 album by Our Lady Peace * "Curve" (song), a ...

, sight distance, and the lengths of crest and sag vertical curves. Roads with higher design speeds require wider

lane In road transport, a lane is part of a roadway that is designated to be used by a single line of vehicles to control and guide drivers and reduce traffic conflicts. Most public roads (highways) have at least two lanes, one for traffic in each ...

s, sweeping curves, wider recovery areas, broader

clear zone A run-off-road collision (or roadway departure) is a US term for a type of single-vehicle collision that occurs when a vehicle leaves the roadway. Causes and consequences Contributing factors can include: * loss of control due to excessive or i ...

s, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Roads and streets with lower design speeds require narrower lanes, sharper/tigher 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 everyday use and 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 scalar quant ...

at which a

motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on rails (such as trains or trams) and is used for the transportation of people or cargo. The ...

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 incorporate 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 60 mph design speed except for a curve with a 45 mph design speed, the entire segment would have a 45 mph design speed. In reality, the road may have a 45 mph 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 ''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." A majority of 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 5 mph, 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