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24 before treatment versus 16 percent after treatment) and seg- · The quality of the existing AMF, based on the information ment locations (i.e., 38 percent before treatment versus 16 described in Chapter 2 of this report; and percent after treatment). · The availability of a group of similar but untreated inter- sections to be used as a reference group. Signalized Intersection Treatments in Urban Areas The original Winston-Salem documentation did not in- clude information on before-treatment-period and after- Description of Treatment and Crash Types treatment-period AADTs. Research team members traveled of Interest to Winston-Salem and extracted the AADTs for each treat- This analysis examined the safety impacts of four urban ment intersection approach for each before-treatment and safety treatments implemented at signalized intersections in after-treatment year from AADT books available at the City Winston-Salem, North Carolina. The treatments were the of Winston-Salem DOT. following: Winston-Salem does not have a computerized intersection inventory that is linkable to crash records. Thus, they could not · Modification of left-turn signal phase (three combinations), provide the research team with data to be used in the · Conversion of nighttime flashing operation to steady development of a reference group of similar untreated sites. operation, After consideration of other alternatives, a reference group was · Replacement of 8-in. signal heads with 12-in. heads, and manually developed. The Winston-Salem Traffic Engineer and · Replacement of single red signal head with dual red signal his staff identified 75 untreated signalized intersections that heads. were similar to the treated sites in terms of traffic volume, num- ber of legs, number of approach lanes, and other characteristics The basic objective was to estimate the change in target during the study period (1990 to 2004). Crash data for all crashes for each of the treatments. Target crashes, which dif- crashes in Winston-Salem for the full study period were ex- fered depending on the treatment, included left-turn crashes, tracted from the North Carolina DOT crash files, which contain nighttime angle crashes, and right-angle crashes. However, data on all crashes statewide, and these data were manually since the treatment might increase other types of crashes matched to the reference intersections based on street names (e.g., the conversion back to regular nighttime phasing could found on the crash reports. AADT data for each year in the full increase rear-end crashes on the major road), additional crash period were then extracted for each reference intersection from types and total crashes were examined. The specific crash types the Winston-Salem AADT books noted above. After eliminat- for each treatment are presented below. Appendix E provides ing intersections lacking in AADT or other data, 60 untreated the details associated with this evaluation. intersections were available for developing the required SPFs. Data Used Methodology Unlike many other jurisdictions, the City of Winston- The general analysis methodology used was the EB before- Salem has documented the installation records for a large after analysis previously described. SPFs and annual correction number of urban safety treatments implemented at intersec- factors were successfully developed for total crashes. Efforts at es- tion and non-intersection locations and has systematically timating SPFs for specific crash types were not successful. Hence, conducted simple before-after studies of those treatments. the proportion of crashes for each crash type and a recalibrated Documentation exists for over 70 individual treatments or over-dispersion parameter were used in the EB analysis. combinations of treatments installed since the 1980s, along with target and total crash counts for before-treatment and Results after-treatment periods of 3 to 5 years. The City of Winston- Table 11 presents the results of the EB analyses for each of Salem provided these files to the research team. The team the four treatment types. For each treatment, results are then chose the four treatments for evaluation based on the presented for both the primary target crashes and for other following: important crash types. Statistically significant results are indicated by asterisks. · A statistical analysis of available crash sample size to ensure the possibility of statistically significant results; Modification of Left-Turn Phase · The timeliness of the treatments to ensure a reasonable current driver and vehicle population (i.e., treatments in Three types of left-turn phasing treatments were identi- 1994 and later); fied. In all cases, the target crashes for these treatments were
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25 Table 11. Crash frequency AMFs for urban signalized intersection treatments by treatment type. No. of AMF Treatment Type Treatment Crash Type (standard error) Sites Replace permissive left-turn phasing with Left-Turn 0.978 (0.277) 3 permissive/protected (1) All 1.045 (0.135) Replace permissive left-turn phasing with Left-Turn 0.021 (0.021)** 8 protected (2) All 0.975 (0.085) Replace permissive/protected left-turn phasing Left-Turn 0.000 (0.006)** 4 with protected (3) All 1.020 (0.123) Replace permissive or permissive/protected with Left-Turn 0.014 (0.014)** 12 protected (combination of 2 and 3) All 0.992 (0.070) Nighttime Angle 0.659 (0.180)* Convert nighttime flash to normal phasing (4) 12 All Nighttime 0.651 (0.145)** Right-Angle 0.580 (0.070)** Replace 8-in. signal heads with 12-in. heads (5) 26 All 0.970 (0.060) Right-Angle 1.050 (0.130) Add second signal head (6) 8 All 1.180 (0.110) ** Statistically significant at the 0.05 significance level * Statistically significant at the 0.10 significance level identified as those involving at least one left-turning vehicle If this is the case, the implication is that this is still a very on the treated roadway. The first treatment involved replac- safety-effective measure from a total-harm perspective since ing a permissive left-turn phase with a permissive/protected left-turn crashes tend to be of the side-impact variety and phase at three sites. There was little change in either the therefore are more severe than rear-end crashes. The results target or total crashes, but the small sample size suggests cau- also imply that the treatment would be most effective overall tion in concluding "no effect." where there is a relatively high frequency of left-turn crashes. The second treatment involved replacing a permissive left- turn phase with a fully protected phase at eight sites. Here, Conversion of Nighttime Flashing Operation the target left-turn crashes were reduced by approximately to Steady Operation 98 percent, a statistically significant reduction, and total crashes changed very little. There were 12 intersections where nighttime (9 p.m. to The third treatment type involved replacing a permis- 6 a.m.) flashing operation was replaced with regular phasing. sive/protected phase with a fully protected phase at four sites. The EB analysis indicated that nighttime angle crashes (the Here, the left turn crashes were eliminated while the total ones most likely to be positively affected) were reduced by crashes changed little. approximately 34 percent, a statistically significant reduction Since the results of the second and third treatments refer to at the 0.10 level of significance. Total nighttime crashes also conversions to a fully protected phase and since the results saw a significant reduction of approximately 35 percent were similar, they were combined into one group to increase the sample of sites to 12. As shown, the combined AMF was Replacement of 8-in. Signal Heads with 0.014 for the left-turn crashes (a statistically significant 12-in. Heads result), and the total crashes again were virtually unchanged. Since the left-turn crashes decreased substantially and total There were 26 intersections where 8-in. signal heads were crashes did not, it is evident that there must have been an changed to 12-in. heads. The EB analysis indicates that right- increase in non-left-turn crashes of the same order as the angle crashes experienced a statistically significant decrease of decrease in left-turn crashes. Unfortunately, these data did approximately 42 percent. Total crashes experienced virtually not allow the research team to examine changes in other spe- no change. This implies an increase in non-angle crashes of cific crash types. Further research is necessary to determine approximately the same size as the decrease in angle crashes. the specific reasons for the effect on non-left-turn crashes. While it is not possible to determine the specific crash types However, it seems reasonable to speculate that introducing a for the non-angle crashes, one could hypothesize that they are protected left-turn phase tended to increase rear-end crashes predominately rear-end crashes, which might be increased by more than others because of the increased number of phases more drivers stopping rather than proceeding through the (and therefore dilemma zone opportunities) and the increase signal--the same effect seen for red-light cameras at intersec- in queues that can result from the reduced green time avail- tions. Since a "tradeoff" occurred and since the severities of able for all traffic not protected by the introduced phase. angle and non-angle crashes can differ, an economic analysis