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.' 'I l ;1 12 determination of whether specific accidents coded as occurring at dusk or dawn hap- pened during a night condition when artificial illumination conld have been of value. The actual amount of night traffic on each route needed to be calculated so that accident rates per million vebicle-miles (MVM) could be separately computed for day and for night travel. The method used to establish the cutoff point between naturai daylight and night when artificial street lighting was needed had to apply to any location. It had to account for latitude and longitude, regardless of season. Artificial light is normally not needed immediately after sunset (or before sunrise). It becomes clearly necessary prior to the time of civil twilight, defined as the time at wbich the center of the sun's disk is 6 deg below the local horizon. The time of civil twilight occurs between 30 and 40 min after sunset (or before sunrise) and is close to the point where natural daylight is nearly undiscernible. Civil twilight was used as a guide in a series of tests that were'made in the Chicago area to measure the HFC decay after sunset or its buildup before sunrise. The results of part of the twilight measurements are shown in Figure 2. Tbis shows the change in ambient illumination after sunset in January, April, and July. These times represent the 3 near-extreme conditions of sunrise and sunset daylight rate of change. In the threshold area, the curves are separated by only ab011t 2 min. For lighting purposes, it was' assumed that the threshoid point iay' midway between sunset-sunrise and civil twilight. Tbis point was originally estimated by visual checks. Figure 2 shows ambient values of 1.5 to 4.9 HFC at tbis point. A change of only 6 min ('rio hour) produces a drop to a level of 1 .or 2 footcandles for even the higher value. In this immediate area then, a point of ambient illumination is reached when artificial lighting levels typically used on roadways (0.5 to 1.5 HFC) represent a signif- icant added factor in driver visibility. The same results were found for a sunrise condition, with even less difference among the seasonal light change values. The range in ambient illumination at the "as- sumed dark" point was 0.9 to 3.3 HFC. A close approximation of the threshold point occurs 15 min before sunrise and 15 min after sunset. Checks were made with moderate to heavy cloud cover, but only about a 5-min variation was found in the basic assumed dark threshold point. Estimates were also made on extremely cloudy days with precipitation. Variation in threshold time was found to be only about 10 to 15 min. Because relativeiy few days of the year have this extreme condition, it was felt this should not make a significant difference. Accident reports seldom reflect the time of an accident closer than 5- min. Traffic volumes are not perfectly spread during each minute of the threshold hours. NIGHT TRAFFIC From the volume studies, on an hour-by-hour basis, the finding was made that 25 percent of urban freeway traffic consistently moves at night. Latitude, longitude, local DST practice, and metropolitan area size appear to cause no significant variation. This is an important finding, for it allows direct calculation of rate ratios in the absence of traffic counts. The night-day ratio (per million vebicle-miles or any other travel exposure measure) is mathematically equal to ~ times the number of night acci- dents divided by the number of day accidents. LIGHTING MEASUREMENTS AND VARIATIONS Most measurements of existing lighting were taken under "live" traffic conditions between 2:00 and 5:00 a. m. Measurements were made at 14 freeway locations. These were chosen to be as representative as possible of typical spacing for each route. Read- ings at 10 sites were taken directly below 2 to 7 adjacent luminalres. For each location these "below-luminaire" HFe readings were tabulated and averaged. Grid readings were then taken between the pair of luminaires that best represented an average. Points were chosen along pavement edges and at quarter-spacing transverse lines. From the grid readings, typical in-service illumination and uniformity ratios were determined. The initial design values were calculated from manufacturers' photometric r..,'""',.._....,,_.,.._~.,..-,-.._. ,".!~