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j , i ;1 ;) :,j il; 'I i 20 I , the summary data would be usable from 14,000 Chicago and Detroit accidents, as gathered in the public lighting needs study. When a correlation between calculated and measured values was not found, it was decided to not use any accident data from lighted routes, unless field measurements were available. Twenty-two route sections were usable for the basic illumination analysis. These are given in Table 6. The day and night accident rates, the night-day ratios of these rates, and the results of illumination measurements are also given. Table 7 gives a comparisnn of lighted and unlighted groups. Rate ratios are given for all accidents and for only fatal and injury accidentS. Rates for all accidents are shown in Figure 3. The sections with a lighting level between 0.3 to 0.6 HFC had the best ratio of night-day accident rates. A chi-square test established that this group had a very significanily lower ratio (the probability of this being a chance finding is less than 1 in 1,000). Testing of only fatal and injury accidents resulted in a similar finding. The sections with higher illumination values appear to level off at an average ratio between 1.6 and 1.7. The higher illumination group was tested against the un- lighted group, and no significant difference was found. The chi-square values, however, suggest that there is a utility in the higher illuminated freeways, as compared with un- lighted ones, even though the data did not allow significant differences to be established. The ~ata do not allow specification of the optimum illumination level from the stand- point of accident reduc,tion. No study sections were available with higher illumination such as 2.0 or 2.5 HFC. The findings do not imply that such higher levels have utility, nor do they disprove the possibility. If a relation exists between uniformity (either average or maximum to minimum), it was not identified in the research. The failure to find any relation is understandable in view of the illumination variations found along the subject routes. , " i1 i EFFECT OF LATITUDE The northern area routes tended to have high illumination values, and the southern routes had mixes of high and low levels. The southern locations were, therefore, ex- amined separately. As a group, the lighted routes showed a better accident rate ratio than the unlighted (DeKalb County, Georgia) route. Also, those lighted sections with lower illumination had significantly better ratios than the lighted sections with higher levels. Thus, the presence of a geographic bias on the illumination levels has no apparent effect on the overall statis,tical findings based on data from both north and south locations. COST-BENEFIT ANALYSIS For the urban data as a whole, the average annual number of day accidents ranged from 12 to 160 per mile of 4-lane lighted freeway, with an average of 74. For 6-lane routes the range was 4 to 295, with an average of 52; for 8- and IO-lane routes, the range was 28 to 197, with an average of 89. These values have been used to compute the expected number of average night accidents for urban freeways with and without lighting. The average accident coslS have been compared with estimated typical free- way lighting coslS, including installation plus 20-year maintenance and energy cost pro- jections. The calculations result in favorable cost-benefit ratios of 2.3 for lighting 4- lane urban freeway sections, 1.4 for 6-lane sections, and 1.7 for 8- or IO-lane sections. ACKNOWLEDGMENT This paper has been prepared to summarize a portion of the findings of a project on the relation between illumination and freeway accidents. This 3-year study was com- menced in 1967 and was financed principally by the illuminating Engineering Research institute, with aid from the Automotive Safety Foundation. REFERENCE 1. Public Lighting Needs. llluminating Engineer, Sept. 1966, pp. 585-602. $