Road Design: Key Concepts and Traffic Calming
Roadway Classifications and Design Elements
- Limited-access facilities: For motor vehicles with the highest speed limit, no direct access to abutting land uses is permitted.
- Arterials: Facilities designed primarily for through movement but permit some access to neighboring lands.
- Collector communications: Designed for through movements and also to provide access to neighboring areas.
- Local communications: Designed mainly to provide access to neighboring areas.
- Communication space: All the space necessary to build the road and the sidewalk.
- Main traffic precinct: The place where cars are used; no pedestrians allowed.
- Side traffic precinct: The place where pedestrians can walk safely.
Curve Design Considerations
This condition means that the minimal radius of the plain circle arc must be R ≥ 800 m or at least R ≥ 0.375 V2n.
Clothoid: A curve in which the radius of the osculating circular decreases with the length of the curve.
Sight Distance Requirements
Stopping sight distance: The minimum sight distance required by the driver to stop the car before it hits an object on the highway. It is of primary importance to the safe working of a highway.
Overtaking sight distance: Of central importance to the efficient working of a given section of highway, especially on single carriageways. Full overtaking sight distances are much larger in value than stopping sight distances. Therefore, economic realities dictate that they can only be complied with in relatively flat terrain where alignments, both vertical and horizontal, allow the design of a relatively straight and level highway.
Factors:
- The perception and reaction distance
- Overtaking time
- The braking distance
- Safety time
- The safety distance
- Closing time
Summit and Sag Curves
The main difference is that one is concave (summit) and the other is convex (sag). The summit is the point which is in the highest point of a concave curve. The sag is the point which is in the lowest point of a convex curve. The minimal radii are derived both for convex and concave curves from the minimum necessary length for stopping in front of an obstacle. The greater curve is better (sometimes radii of 10,000m or more are used on motorways).
Cross-Section Factors
- Roadway width
- Difference in altitude between the level of terrain and vertical alignment
- Embankment
- Cut
- Gradient of slopes 1:n
- Type of ditch, drainage channel, and gradient of terrain
Roadway Capacity and Level of Service
Capacity: The maximum sustainable flow rate at which vehicles or persons reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a specified time period under given roadway, geometric, traffic, environmental, and control conditions; usually expressed as vehicles per hour, passenger cars per hour, or persons per hour.
Level of service: A qualitative measure describing operational conditions within a traffic stream and their perception by drivers and/or passengers.
Traffic Calming Measures
Traffic calming: Combination of mainly physical measures that reduce the negative effects of motor vehicle use, alter driver behavior, and improve conditions for non-motorized street users.
- Physical elements
- Psychological elements
- Traffic calming elements within intersections – both physical and psychological
Physical Traffic Calming Measures
- Narrowing traffic lanes
- Traffic circles (roundabouts)
- Chokers
- Speed humps/bumps
- Curb extensions
- Raised crosswalks
- Central narrowing islands
- Raised intersections
- Chicanes
- Diagonal diverters
Psychological Traffic Calming Measures
- Traffic signs
- Repeated traffic signs
- Horizontal marking on pavement
- Illuminated traffic signs
Typical Railway Track Arrangement
In a typical railway track arrangement, there are:
- Screw spike
- Rail
- Dowel
- Concrete sleeper
- Elastic tie plate
- Bridge plate
- Clip