Design speed
Encyclopedia
The design speed is a tool used to determine geometric features of a new road
during road design. The design speed chosen for a highway is a major factor in choosing superelevation rates
and radii of curves, sight distance
, and the lengths of crest and sag vertical curves. Roads with higher travel speeds require sweeping curves, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Lower speed roads can have sharper curves, less banking, less sight distance, and sharper hill crests and valleys.
While a road's design speed is sometimes used to determine an initial speed limit, it is an imperfect measure of the maximum speed
at which a motor vehicle
can be operated for reasons including but not limited to:
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".
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 attemtps 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 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 speeds appropriate for the neighborhood.
Road
A road is a thoroughfare, route, or way on land between two places, which typically has been paved or otherwise improved to allow travel by some conveyance, including a horse, cart, or motor vehicle. Roads consist of one, or sometimes two, roadways each with one or more lanes and also any...
during road design. The design speed chosen for a highway is a major factor in choosing superelevation rates
Cant (road/rail)
The cant of a railway track or a road is the difference in elevation between the two edges...
and radii of curves, sight distance
Stopping sight distance
Stopping sight distance is a term used in highway design. It is defined as "the length of roadway ahead visible to the driver."-Sight distance:...
, and the lengths of crest and sag vertical curves. Roads with higher travel speeds require sweeping curves, steeper curve banking, longer sight distances, and more gentle hill crests and valleys. Lower speed roads can have sharper curves, less banking, less sight distance, and sharper hill crests and valleys.
While a road's design speed is sometimes used to determine an initial speed limit, it is an imperfect measure of the maximum speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
at which a motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...
can be operated for reasons including but not limited to:
- It is only a theoretical or laboratory measurement created before a road is even built.
- 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 an 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 speed limitSpeed limitRoad speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...
caps, so it would not be legal to sign some roads at their design speeds.) - It is based 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".
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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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 attemtps 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 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 speeds appropriate for the neighborhood.
Factors
When roads are planned, the selected design speed may be based on or influence several factors, including but not limited to:- geometric design of road features
- planned operating speedOperating speedThe operating speed of a road is the speed at which motor vehicles generally operate on that road.The precise definition of "operating speed", however, is open to debate. Some sources, such as the AASHTO, have changed their definitions recently to match the common use of the word...
- legislated speed limit caps
- anticipated traffic volume
- the road's functional classificationFunctional classificationThe functional classification of a road is the class, or group, of roads that the road belongs to. There are three main functional classes as defined by the United States Federal Highway Administration: arterial, collector, and local.- Arterial roads :...
See also
- Geometric design of roadsGeometric design of roadsThe geometric design of roadways deals with the positioning of the physical elements of the roadway according to standards and constraints. The basic objective in geometric design is to provide a smooth-flowing, crash-free facility. Geometric roadway design can be broken into three main parts:...
- Operating speedOperating speedThe operating speed of a road is the speed at which motor vehicles generally operate on that road.The precise definition of "operating speed", however, is open to debate. Some sources, such as the AASHTO, have changed their definitions recently to match the common use of the word...
- Speed limitSpeed limitRoad speed limits are used in most countries to regulate the speed of road vehicles. Speed limits may define maximum , minimum or no speed limit and are normally indicated using a traffic sign...
- Traffic calmingTraffic calmingTraffic calming is intended to slow or reduce motor-vehicle traffic in order to improve the living conditions for residents as well as to improve safety for pedestrians and cyclists. Urban planners and traffic engineers have many strategies for traffic calming...