The goals of lighting systems for public thoroughfares used by vehicles and pedestrians include:
Providing comfortable levels of illumination that allow people to quickly and accurately identify objects on the roadway.
Improving traffic safety and traffic movement under a variety of weather conditions.
Supplementing vehicle headlight illumination by enabling drivers to see details better, determine their location and react to a traffic conditions on or near the roadway.
Helping pedestrians negotiate their designated pathways and recognize the presence of other pedestrians, vehicles, and objects in their vicinity.
Minimizing shadows and reducing direct and reflected glare.
Controlling light trespass and light pollution.
Expressing the design goals in terms clearly understood by lighting designers and DOT planners, engineers and administrators.
Well designed street and roadway lighting can reduce nighttime accidents, aid in police patrolling, improve traffic flow, promote the use of retail, business and industrial facilities at night, and promote urban pride and a sense of wellbeing after hours.
NOTE: The specification of a street, roadway or area lighting system must take into account a wide variety of local conditions, luminaire specifications, and illuminance calculations. What follows is a summary of the major lighting design criteria as presented by the IES. Consult the 10th Edition Lighting Handbook and the IES RP-8-00 Standard Practice for Roadway Lighting, for complete design data and all pertinent calculation tables.
Design of street and roadway lighting systems takes into consideration a variety of factors such as visibility, economics, aesthetics, safety and environmental. The design process follows these major steps:
1. Determine the Roadway Classification. The IES uses the following classifications to designate street, roadways and other thoroughfares having sufficient visual complexity to require nighttime illumination:
Freeway. An access controlled toll or non-toll divided major roadway with no crossings at grade.
Freeway Type A: These have high traffic volume, are found in metropolitan or urban core areas and operate at near capacity during the early hours of darkness.
Freeway Type B: These include all other controlled access divided roadways where illumination is required
Expressway. Major divided roadway for through traffic with partially controlled access. There are interchanges at major crossroads.
Major. These are principal networks for through traffic flow. They connect areas of heavy traffic generation and major highways coming into the city core from rural areas. They are sometimes categorized as “primary” and “secondary.”
Collector. These collect traffic from major roadways and feed it to local streets that serve industrial, commercial and residential areas.
Local. These are the streets that provide access to and around urban and suburban abutting residential, commercial and industrial areas. They typically do not handle through traffic. They are illuminated by a wide variety of luminaire types. They are often intersected by collector roadways.
Interchange. These are grade separated combinations of overpasses or underpasses and ramps that permit entrance to and exit from roadways.
Intersection. This is the point where two or more continuously lighted roads cross one another on the same grade.
Alley. These are narrow public ways with a block of abutting properties usually providing access to the rear of the properties.
Sidewalk. Paved or otherwise improved areas for pedestrian use, located within public rights-of-way and vehicle traffic ways.
Pedestrian walkway. A public pathway for pedestrian foot traffic usually not within vehicle rights-of-way. Examples: pedestrian overpasses, pedestrian tunnels, access walkways into parks or plazas, midblock street crossings, etc.
Bikeway (Bicycle Lane and Bicycle Trail). This can be any road, street or pathway specifically designated for bicycle traffic. Bicycle lanes are designated portions of a motor vehicle road. Bicycle trails are separate trails or paths for cyclists, sometimes shared with pedestrians, where vehicle traffic is prohibited.
2. Determine the Area Classification. Lighting design will be guided to a certain extent by the areas through which the thoroughfare passes:
Commercial: Business districts in a community where there will be high volumes of nighttime vehicle and pedestrian traffic.
Intermediate: Urban areas with heavy pedestrian traffic, for example the areas around libraries, shopping esplanades, recreation centers, town squares, retail clusters, large apartment complexes, etc.
Residential: Areas with single or multi-family homes, town houses, condos, and small commercial establishments. Streets are usually laid out in a grid pattern.
3. Determine the Pavement Luminance. This requires collecting data regarding the reflectance characteristics of the materials used on the road or pathway surface. Reflectance will affect the quality and uniformity of the lighting.
4. Determine Light Distribution Classifications. This involves an understanding of the wide variety of available light sources and luminaires, their classification and suitability for lighting different types of roads, streets and ways, The light from the luminaires must be:
Proportionate to the dimensions of the roadway being illuminated, the space between the light sources and their mounting height
The light distribution from luminaires is calculated based on three general criteria:
Vertical light distribution
Lateral light distribution
Control and cutoff of the light distribution above maximum candlepower
5. Determine Lighting System Geometries.
Mounting heights of roadway luminaires have increased over the last few decades, primarily due to higher output, more efficient light sources. Mounting heights of 40 feet and above are common along roadways allowing for more uniformity of illuminance with high wattage lamps.
High Mast Interchange luminaires exceed 60 feet in height. They consist of clustered multiple high-wattage luminaires (3-6) and utilize mechanical lowering devices for maintenance.
Over the same period, there has been a trend to lower the mounting heights of luminaires in urban and residential areas, These luminaire designs provide improved aesthetics and visual comfort and complement the improved efficiency and visual quality of advanced, “white light” sources such as LEDs and CMH. An example is the return to the use of pole top luminaires. New products like the SYLVANIA Pole Top LED Retrofit kit allows luminaires that currently use HPS or MH light sources to be easily converted to LEDs.
6. Determine Pole Spacing.
This is influenced by location of existing utility poles or masts, lengths of blocks, property lines and the geometry of the street or thoroughfare being illuminated.
Generally it is more economical especially with roadways, parking areas to use light sources with higher lumen output at longer intervals and at higher heights, provided the mounting height is within the light distribution range of the luminaires.
Other factors to consider are: access to luminaire for maintenance, potential for collision with poles, system glare aspects, visibility of signs and signals, aesthetics, interference with trees and use of poles for other functions (e.g. traffic lights)
The final design of street and roadway lighting systems involves a whole host of variables and calculations not covered in this summary. Local topography, system depreciation and maintenance requirements, merging, diverging and converging traffic lanes, underpasses, complex interchanges, under and overpasses, tunnels, curbs, medians and sidewalk characteristics are just a few of the conditions that will affect the final design. Consult IES publications for more in depth coverage of these and other design topics.