Sag curves in night-time conditions
A critical design concern for sag curves during night-time conditions can be
headlight sight distance, where the length of the highway illuminated by the car’s
headlights is the governing parameter. The critical measurement in this instance
will be the height of the headlights above the surface of the highway. This
process is, however, highly sensitive to the angle of upward divergence of the
light beam.
The governing formulae are:
(6.69)
(6.70)
where
Hl is the height of the headlight above the highway in metres, normally assumed
as 0.61 m.
Sis the required sight stopping distance in metres, dependent on design speed
bis the inclined upward angle of the headlight beam relative to the horizontal
plane of the vehicle (in degrees).
6.7 References,
Bannister, A. & Raymond, S. (1984) Surveying. Longman Scientific and Technical,
Harlow, Essex, UK.
DoT (1993) Highway Link Design. Departmental Standard TD 9/93.Design Manual for
Roads and Bridges, Volume 6,Road Geometry. The Stationery Office, UK.
DoT (1996) Cross-sections and Headrooms. Departmental Standard TD 27/96.Design
Manual for Roads and Bridges, Volume 6,Road Geometry. The Stationery Office,
London, UK.
O’Flaherty, C. (1986) Highways: Traffic Planning and Engineering,Vol. 1. Edward
Arnold, London.
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Geometric Alignment and Design 191
