Section 4: Conventional vs. High Mast LightingAnchor: #i1004424
Conventional lighting systems are those using mounting heights of 50 feet or less. This description is used to differentiate between conventional mounting heights and high mast lighting, which uses mounting heights of 100 feet or more.
This section compares the advantages and disadvantages of conventional and high mast lighting and provides guidelines for deciding when to use high mast lighting.Anchor: #i1004441
Installation cost comparisons between high mast and conventional lighting systems vary widely, depending on the application. High mast lighting for interchanges is frequently less expensive to install than conventional lighting, due to reduced complexity of conduit and conductor and the smaller number of fixtures and poles required. Outside the interchange, conventional lighting usually requires a smaller initial cost.Anchor: #i1004451
Maintenance costs for the two types of systems differ greatly.
Conventional lighting requires the use of a bucket truck and frequently requires extensive traffic control, such as signs, cones, and lane closures. When poles are mounted on concrete traffic barriers (CTBs) or single slope concrete barriers (SSCBs), the inside lane usually has to be closed, resulting in significant traffic disruptions.
One or two persons with a pickup truck can usually perform maintenance on a high mast lighting system. High mast lighting may also eliminate the risks involved with having personnel working near high-speed traffic.Anchor: #i1004471
Some Deciding Considerations
Some important questions to consider when deciding whether to use conventional or high mast lighting are:
- Will lane closures be necessary for maintenance? What will be the effect on traffic?
- What is the initial cost difference?
- What is the maintenance cost difference? (When designing a lighting system, consider all maintenance costs, including costs incurred by other governmental bodies.)
- Would future upgrading of the roadway require relocating a conventional lighting system? Can a high mast system be installed that will not require relocation and that can provide construction lighting for future roadway projects?
- What is the proximity of airports and residential areas?
High Mast Systems Recommended
High mast lighting should be considered for most urban interchanges that qualify for complete interchange lighting and for tangent sections of freeways with initial average daily traffic (ADT) of 70,000 or greater where lane closure would be necessary for the maintenance of a conventional lighting system and where a study shows that substantial traffic flow disruptions would occur during such lane closures.Anchor: #i1004516
High Mast Systems and Light Trespass
High mast lighting should not be installed in areas where light trespass is an issue. This is generally areas where residential development is located directly along the highway right-of-way and pole placement is limited to the outside of the roadway. Special high mast designs should be considered to avoid light trespass, if high mast lighting must be employed. See Glare and Sky Glow Issues of this chapter for more information on light trespass.Anchor: #i1004529
High Mast Design and Layout
Layouts for high mast lighting are much more complicated than those for conventional lighting, since there are many different lighting fixtures and schemes available. The Traffic Operations Division (TRF) provides high mast illumination design assistance upon request. High mast lighting design procedure is beyond the scope of this manual.
Remember, high mast lighting layouts should include soil boring logs. See the Geotechnical Manual (Bridge Collection) for details.
Figure 6-4 shows a typical high mast plan layout for safety lighting.
Figure 6-4. Typical high mast plan layout for safety lighting.