What are radials?
In construction the word radials carries two meanings: the radian, a unit of angle used in calculation, and radial streets, the roads running outwards from a city centre. Both come from the Latin radialis, from radius. Which one is meant is usually obvious from context: an engineer calculates in radians, a planner draws radials on a street map.
The radian as a unit of angle
One radian is the angle subtended by an arc equal in length to the radius of the circle. That definition needs no arbitrary subdivision, unlike the degree with its 360 steps.
| Angle | In radians | In degrees |
|---|---|---|
| Full circle | 2 pi | 360 |
| Half circle | pi | 180 |
| Right angle | pi/2 | 90 |
| 1 radian | 1 | about 57.3 |
To convert degrees to radians, multiply by pi/180; the other way round, multiply by 180/pi. Radians turn up in structural calculations, in slopes and rotations, and in spreadsheets, CAD and CAM, where trigonometric functions work in radians as standard. Feeding degrees into a spreadsheet sine or tangent without converting produces wrong answers with no error message to warn you.
Radial streets in town planning
In town planning, radials are the streets that run outwards from the centre. They are almost always paired with ring roads. Together they form the radial-concentric pattern, sometimes called a radial access system: radials carry traffic to and from the centre, rings carry traffic around it.
- Radials give access to the centre. They connect the inner city directly with the suburbs and the wider road network.
- They concentrate traffic. Because every radial converges on the same point, loading increases towards the middle.
- Ring roads relieve the radials. Traffic with no business in the centre is diverted onto the ring.
- Without a ring, radials overload quickly. The centre becomes the bottleneck for through traffic that does not need to be there.
Many old cities show this pattern naturally. The radials follow the historic approach roads, and the first ring sits on the line of the demolished fortifications.
Related terms
- Radius
- Arc radius
- Degrees
- Ring road
- Access road
- Street network
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