The Acu_Calc program generates a typical roadway lighting layout using the specifications provided in the input file. One of these is the overhang/setback. For this program, the overhang and setback refer to the horizontal distance from the edge of the travelled way to the luminaire, measured perpendicular to the edge of the travelled way. If the luminaire is over the travelled way, the Overhang value is positive. Likewise, a distance that a luminaire is located "back from" the edge of the travelled way is setback, and should be entered as a negative value for Overhang.
The default for the program is that the same overhang/setback value is applied to all the luminaires along both sides of a single roadway (corresponding to all the lumianires within the blue box in the upper diagram.) A "single roadway" means that all lanes are adjacent, even though they may be travelled in different directions. The default direction of travel used in Acu_Calc (indicated in the gold box in the diagram) arises from the North American, drive-on-the-right pattern, but two-way traffic can be evaluated if the luminaires' distributions are laterally symmetric. For a single roadway, the program will apply the same overhang/setback to luminaires located on either side of the roadway.
The program can also evaluate lighting for roadways that are separated by a median. When a positive value for median width is specified, the program creates a second set of adjacent lanes, equal to the number specified but located across the median (shown in the green box in the lower diagram.) The second part of the roadway is assumed to be illuminated by a lighting system (in the green box) that mirroAC the equipment over the calculation grid portion of the roadway, with the mirror line along the middle of the median.
For such geometry, the overhang/setback for luminaires in the median (in the red box in the lower diagram) may differ from those not on the median, on the outside of the roadway. To allow for such situations, Acu_Calc uses a second overhang value for all luminaires located on the median, called OverMedian.
OverMedian is applied to all the luminaires located between the two sections roadway (in the red box): those on the left-hand side of the right-hand roadway and also on the median side of the other roadway. The OverMedian distance is measured from the edge of the travelled way, with positive values for luminaires over the travelled way, the same way as Overhang.
This separate specification allows for consideration of lighting systems that illuminate two roadways in the same right-of-way. The illumination model for each roadway should include any contribution from luminaires that are primarily lighting the median or other roadway.
In summary, for the Acu_Calc program, the overhang and median values are related. The default is that the roadway does not have a median and that the luminaire overhang is the same on each side of the roadway (in the blue box.) When the median is specified with a positive value, the Overhang value is applied to those luminaires not on the median, and the OverMedian value is applied to the luminaires on the median (in the red box.)