Glare is caused by Harsh uncomfortably bright light. It’s a sensation in response to light that can take a number of forms. Results from bright, steady, dazzling light or from shiny surfaces. Can also caused by very bright foreground or by light reflected by fog or snow. Glare affects both day & night driving performance. Effects of glare on drivers are classified as Discomfort glare and Disability glare. Disability glare is also known as physiological glare. It is created by bright light. Its comes from light scattering in the ocular media. Reduce driver’s ability to perform visual task. Glaring light scattered in the eye can be expressed as superposition of uniform luminance onto retinal image. Discomfort glare is caused by bright light. It cause discomfort or annoyance. Scale of discomfort glare was used by DeBoer. It is influenced by 3 factors. They are location of glare source, luminance of background and luminance & size of glare source. This civil engineering seminar topic gives an insight into causes of Glare during Night Time Driving.
Headlamp Glare - Vehicle headlamps illuminate the area ahead of vehicle and overhead signs. Essential part of roadway visibility system. Must provide sufficient lighting in the field of view of a driver. Headlamps need to produce and project high light levels.
Causes of Headlight Glare
1) Illumination from the glare source
2) Glare angle
3) Back ground luminance
4) Size of the glare source
5) Glare source luminance
5) Driver age
6) Reflective surfaces
7) Glare from mirrors
Glare On Two Lane Highways are caused by Lower Light levels, Oncoming traffic closer to driver’s line of sight, Complex Roadway geometry, Less restricted roadway access, Less roadway markings and Closer proximity of pedestrians. Visibility of an object depends on overall light levels. Pole mounted roadway lighting help to see on unlighted roads. Visibility depends on the contrast of an object against its background. Glare reduce visibility. Can be controlled by higher light levels on road. Glare is much larger on unlighted roads.
Discomfort from headlamp glare increases when visual task is more difficult. Chances for hazard along a two lane highway is greater. Vehicles enter from variety of locations and directions. Not restricted for bicycle travel or pedestrian use. Difficult and less safe driving. Multi-lane highways are marked with indications for traffic lanes. Two-lane highways may not have median lines or nor markings. Roadway markings facilitate efficient and safe driving. No direct impact on driver. Provide visual guidance to a driver. On two lane highways pedestrians more closer to vehicle traffic. Less common on highways but are not restricted. Pedestrians themselves are not always easily seen. Dark cloths have less reflectance. Do not provide much contrast against the roadway pavement. Pedestrians themselves often overestimate how visible they are to oncoming traffic.
To reduce illumination reaching driver’s eye, glare screens are placed in the median of roadway. Cost effective way to reduce glare. Improve safety in temporary work zone. Any type of object with certain width and placed at certain spacings that prevent glare. There are 3 types of screens. Type1 - continous and block light from all angles. Eg: earth mounds and concrete barriers. Type2 - continuous screen of an open material opaque to light coming from angle of 0-20 degrees. Transparent for beyond 20 degrees. Eg: expanded metal mesh, knit polyester fabric and fencing. Type3 - made up of individual elements that block light from angles of zero to 20 degrees. Provide clear visibility beyond 20 degrees. Manufactured by individually supported paddles at intervals.
Glare can be controlled by planting trees or bushes in the median. Position of trees should be fixed. Should be sufficiently away from carriage way. Tree guard should be prepared to protect the young sapling. Prevent glare from sunlight as well as from vehicle headlamps. Ordinary light consists of electromagnetic waves which vibrate in all directions. When passes through polarizing filter light waves are absorbed. Become linearly polarized. Can improve visibility by decreasing the amount of light from oncoming headlights. Both vehicles should be equipped with the proper hardware. Should have a polarizer and analyser or polarizing filter.
AASHTO refers median as a portion of divided highway separating the travelled way for traffic in opposing directions. Provide recovery area for out-of-control vehicles. Provide stopping area. Allow space for speed changes and storage of left turning and U turning vehicles. May be depressed or raised. Increasing median width decreases the effects of glare from opposing headlights. Increasing lateral separation shifts the beam pattern of the opposing vehicle and results in lower intensities of light. Reduce both disability and glare.
Indirect measures to reduce effects of glare are by reducing glare on highways by reducing the need of visible light. Improve night time visibility of pedestrians, lane lines, signs and other objects. UV radiations when absorbed get converted to longer wave length visible light-FLUORESCENCE. It makes objects more easily seen. Improve highway safety. It is not possible to recommend one counter measure that would eliminate discomfort glare for everyone in all situations. Glare at night can be mitigated by design changes in roadway, automobiles, and vehicles lighting systems and thus improve night vision on highways. No of counter measures can be implemented with minimal cost for glare reduction.