ICCTP 2010: Integrated Transportation Systems— Green•Intelligent•Reliable © 2010 ASCE
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Development of a Safety Performance Function for Signalized Diamond Interchange Ramp Terminals
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Heng WANG1, Xiao QIN2 and David NOYCE3
1. Traffic Operations and Safety (TOPS) Laboratory, Department of Civil and Environment Engineering, University of Wisconsin-Madison, 1241 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706; email:
[email protected] 2. Department of Civil Engineering, South Dakota State University, CEH 148, Box 2219, Brooklings, SD 57007; PH 605-688-6355; email:
[email protected] 3. Traffic Operations and Safety (TOPS) Laboratory, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, 1204 Engineering Hall, 1415 Engineering Drive, Madison, WI 53706; PH (608) 265-1882; FAX (608) 262-5199;email:
[email protected] ABSTRACT Interchange ramp terminals are important parts of the highway system. However, crashes that occur at ramp terminals are not well researched. Additionally, very few research efforts have developed safety performance functions for interchange ramp terminals. This research collected data from six none-frontage-road diamond interchanges in Madison, Wisconsin, and developed a safety performance function for signalized diamond interchange ramp terminals by using a generalized linear regression approach. The factors considered include the following: volume, clearance interval (i.e., yellow and all-red intervals), off ramp exclusive right turn phase, and spacing between two adjacent terminals. The major findings suggest three outcomes: (1) the crash frequency will increase with the deficient yellow or all-red intervals; (2) the probability of rear-end crashes increases and the probability of angle crashes decreases if the interchange ramp terminals have an off ramp exclusive right turn phase; and (3) as terminal spacing increases, the crash frequency will increase, with crash type dependent on terminal spacing. INTRODUCTION Interchange ramp terminals are important parts of the highway system. Ramp terminals provide the connection between various highway facilities, such as freeways and arterials. Similar to conventional intersections, traffic flow with different travel directions conflict with one another at ramp terminals, which can increase the crash potential. Interchange ramp terminals contribute to a large portion of crashes within interchange areas. The Fatality Analysis Reporting (FARS) System data shows that approximately 25 percent of fatal crashes within interchange areas are ramp terminal crashes (Torbic et al., 2009). Considering the large amount of interchanges across the country, the number of crashes that occur at ramp terminals is of significance and requires additional research to better understand.
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Crashes which occur at conventional four-leg at grade intersections have been well researched to a certain level. However, it is not safe to transfer the knowledge gained from at-grade conventional intersections to interchange ramp terminals; there are several substantial differences between them. One difference is that conventional intersections and ramp terminals have different pattern of conflict points, as shown in Figure 1. Signal timing plan is another important difference between conventional intersections and diamond interchanges. The signal timing plan of diamond interchanges will consider two ramp terminals together, due to the close spacing of two ramp terminals.
Figure 1. Conflict points at conventional intersections and interchange ramp terminals. Upon realizing the difference between conventional intersections and interchange ramp terminals, and the large amount of crashes at ramp terminals, it is necessary to identify the factors influencing ramp terminal crashes and develop specific safety performance functions (SPFs) for them. This paper will focus on developing a SPF for signalized diamond interchange ramp terminals, considering the factors of volume exposure, clearance interval, off ramp exclusive right turn phase, and spacing between two terminals. LITERATURE REVIEW Only two previous research efforts were identified as trying to build a safety performance function (SPF) for interchange ramp terminals. One was conducted by Torbic and Harwood et al (2007) and the other was conducted by Bushu and Parajuli et al (2006). Torbic developed Interchange Safety Analysis Tools (ISAT) which include 16 SPFs for interchange ramp terminals considering the factors of area type, type of traffic control, number of legs, severity level, and volume on ramps and crossroads. The major deficiency of ISAT ramp terminal SPFs is they were derived from SafetyAnalyst (Harwood et al., 2004) which was developed specific for conventional intersections. Bushu et al. collected data from 380 ramp terminals in the province of Ontario, Canada and developed six different SPFs specific for ramp terminals considering the factor of geometry type, traffic control type, and severity level. In general, previous researches
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considered the impact of different traffic control types and geometric conditions, but did not consider the impact of signal timing and spacing between adjacent terminals. The literature is scarce when considering the safety effects of clearance intervals (i.e., yellow and all-red intervals) for interchange ramp terminals. However, findings for conventional intersections crashes can be traced back to 1985 when Zador et al. (1985) investigated the effect of signal timing on traffic flow and crashes at signalized intersections. Daytime (6:00 am to 8:00 pm) traffic and crash data was received from 91 intersections from eight metropolitan areas around the United States. All 91 intersections were clustered into five groups in terms of their characteristics and crash rates. The results showed that crash rates increased as the adequacy of the clearance intervals decreased. The crash rate for the group with the least adequate clearance intervals was higher than the crash rate for the group with the most adequate intervals. Following researchers confirmed this conclusion. Bonneson and Zimmerman (2006) found that an increase in yellow interval duration of 1.0 s could arouse a 40 percent reduction in crashes. Based on 63 red-light-related crashes, Zimmerman and Bonneson (2005) found that crash type was related to time in the all-red interval of a crash, i.e., left-turnopposing crashes within the first five seconds of red, while right-angle crashes generally after five seconds of red. By using the generalized estimating equations which are an extension of generalized linear models to the correlated data, Wang and Abdel-Aty (2007) concluded that larger values of yellow and all-red intervals will reduce rightangle crashes in the conflict zone near the approach. Since red-light-running violations and conflicts are positively related to red-lightrunning crashes, several research efforts have been focused on the effect of clearance intervals on red-light running violations. Retting et al. (2008) conducted an experiment that increased the yellow interval by about 1.0 s at six approaches to two intersections in Philadelphia, Pennsylvania. Their results showed that yellow timing changes reduced red light violations by 36 percent. Bonneson and Zimmerman’s research confirmed this result (Bonneson et al., 2004). Their finding showed that an increase of 1.0 s in yellow interval, as long as it does not exceed 5.5 s, will decrease the frequency of red-light violation by at least 50 percent. Bonneson and Son (2003) found that the frequency of red light running is at relatively low value for yellow intervals between 3.8 s and 5.0 s, increases significantly for yellow intervals less than 3.5 s and increases slightly for yellow intervals larger than 4.5 s. Gates et al. (2007) analyzed driver behavior in dilemma zones at signalized intersections and confirmed that yellow intervals which were deficient with respect to ITE equation for computing the length of yellow interval contribute to a greater likelihood of red light running. In terms of the safety effects of spacing, previous studies concentrated on the access spacing upstream of on ramp or downstream of off ramp, however, no study has focuses on the safety effect of spacing between two adjacent ramp terminals. NCHRP Report 420 summarizes “many studies over the past 40 years have shown that accident rates rise with greater frequency of driveways and intersections. Each additional driveway increases accident potential.....In general, each additional access point per mile
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increases the accident rate by about four percent” (Gluck et al., 1999). Gluck et al. (1999) suggested the minimum access spacing standards for a four-lane crossroad at interchange. For fully developed urban areas, the suggested spacing for the first access from the off ramp, the first access before the on ramp, and the first major signalized intersection are 230 feet, 300 feet, and 805 feet, respectively. Zhou et al. (2008) suggest that “the state transportation agencies and the traveling public may benefit greatly by an increase in the amount of limited access right-of-way at interchange areas to a minimum of 600 feet and a desirable 1,320 feet”. In summary, previous researches has developed SPFs for interchange ramp terminals, but did not consider the factor of signal timing and terminal spacing which both have effect on crashes. OBJECTIVE AND METHODOLOGY The objective of this research was to develop a SPF for signalized diamond interchange ramp terminals. The major factors considered include volume, clearance interval, off ramp exclusive right turn phase, and spacing between two ramp terminals. A generalized linear regression model was considered as the approach to develop the SPF. Although the generalized linear model has some short-coming related to the potential for over dispersion, it still can give an accurate approximation and most critically, it is easy to understand. DATA COLLECTION Focus Area As shown in Figure 2, each diamond interchange has a pair of ramp terminals which form the focus area of this research. Each ramp terminal has four legs: two legs for crossroad, one for off-ramp and one for on-ramp. This research considered the pair of ramp terminals together.
Figure 2. Study focus Area: diamond interchange ramp terminals. Sites Selection
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The study sites in this research were selected from state highway STH12/14/18/151, often called the Beltline Highway, in Madison, Wisconsin. There are 15 interchanges on the Beltline Highway. Among these, there are seven none-frontageroad diamond interchanges. And six of the seven interchanges have available signal timing data. Therefore, the six interchanges were selected as study sites and shown in Table 1 with their IDs. Table 1. Study Sites and Their IDs. ID Interchange Location 1 Beltline Highway & Greenway Blvd 2 Beltline Highway & Mineral Point Road 3 Beltline Highway & Old Sauk Road 4 Beltline Highway & Rimrock Road 5 Beltline Highway & S. Gammon Road 6 Beltline Highway & US51Highway
Table 2. Crash History of Selected Diamond Interchange Ramp Terminals. 2004 2005 2006 2007 2008 ID RearRearRearRearRearAll Angle All Angle All Angle All Angle All Angle End End End End End 1 10 4 2 6 3 2 6 2 0 5 2 0 5 3 0 2 4 3 0 4 0 2 7 5 1 7 5 1 8 5 2 3 6 4 1 4 3 1 4 3 0 5 5 0 7 4 2 4 7 4 1 7 3 2 8 1 2 6 2 1 6 4 1 5 37 19 12 24 10 10 21 11 6 17 8 4 21 10 8 6 25 16 6 23 11 6 20 9 5 32 11 11 21 12 4 Total 89 50 22 68 30 23 66 31 14 72 33 17 68 38 17 Table 3. Characteristics of Selected Diamond Interchange Ramp Terminals. Yellow All-red Speed Spacing Total AADT AADT AADT AADT Interval Interval Limit ID (ft) crashes ramp1 ramp2 crd1 crd2 (s) (s) (mi/h) 1 32 8100 2000 4200 21100 4.0 1.0 35 480 2 30 9100 10000 32900 36650 3.5 2.0 35 280 3 26 8100 3700 19450 28700 3.5 2.0 35 420 4 34 4300 8000 12300 12300 4.0 2.0 35 490 5 120 14600 6600 45050 50850 3.5 1.5 35 430 6 121 24800 7400 33500 40200 4.0 2.5 45 400 Note: AADTramp1, AADTramp2, AADTcrd1, AADTcrd2 are shown in Figure 2.
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Figure 4. Geometry and signal control comparison between with and without off ramp exclusive right turn phase. Data Collection and Description For each study site, crash data, traffic volume, signal timing, and geometry data were collected, shown in Table 2 and Table 3. Five years of crash data (2004 to 2008) were collected from the Wisconsin Department of Transportation (WisDOT) MV4000 crash database. All intersectionrelated crashes within interchange area were identified as ramp terminal crashes. Average annual daily traffic (AADT) of all off-ramps and crossroads for the six interchanges were obtained from WisDOT traffic volume counts (WisDOT, 2007). The most recent counts were obtained in 2006. It was assumed that the volume almost kept constant over the five year study period. Signal timing plans for all the ramp terminals were obtained from WisDOT and the City of Madison. All six sites use similar signal timing sequences. The off ramp left turns use protected phase only, while crossroad left turns use both protected and permissive phases. The major difference among signal timing plans of the six sites is the off ramp exclusive right turn phase. For the off ramps with exclusive right turn phases, left and right turns are controlled separately by two different signal head (shown in Figure 4). PRELIMINARY ANALYSIS Crash Type and Crash Severity There were 363 crashes which occurred at the six sites during the five year study period. Among the 363 crashes, 239 (65.84%) were property damage only crashes and 124 crashes (34.16%) involved some sort of an injury. As presented in Table 4, rear-end crashes and angle crashes were the two major types of crashes. There were 182 (50.14%)
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rear-end crash and 93 (25.12%) angle crashes. Compared with conventional signalized intersections whose rear-end crashes are approximately 40 percent of total crashes (Yan et al., 2008), interchange ramp terminals have a higher percentage of rear-end crashes. One reason for this is that operating a vehicle at an interchange ramp terminal is more complicated and drivers have more decisions to make at an interchange ramp terminal area than a conventional intersection area. A prior study indicates most rear-end crash at signalized intersections occur when two successive drivers approaching the intersection make conflicting decisions when the yellow signal appears (Yan et al., 2008). With the number of decisions increasing, the possibility that two drivers will make conflicting decisions also increases. Table 4. Movement at Fault for Different Crash Type. Crash Type
Going Straight
81 (44.5%) 50 Angle (53.8%) 13 Sideswipe (29.5%) 9 No Collision (30.0%) 1 Head On (33.3%) 6 Other (54.5%) 160 All Crashes (44.1%) Rear-End
Slowing Left Turn Right Turn Down or Stopped 4 22 71 (2.2%) (12.1%) (39.0%) 33 3 7 (35.5%) (2.2%) (7.5%) 18 3 2 (40.9%) (6.8%) (4.5%) 6 9 3 (20.0%) (30.0%) (10.0%) 2 0 0 (66.7%) (0%) (0%) 1 3 1 (9.1%) (27.3%) (9.1%) 64 40 84 (17.6%) (11.0%) (23.1%)
Changing Lane or Merging 4 (2.2%) 0 (0%) 7 (15.9%) 1 (3.3%) 0 (0%) 0 (0%) 12 (3.3%)
Other
Total
0 (0%) 0 (0%) 1 (2.3%) 2 (6.7%) 0 (0%) 0 (0%) 3 (0.8%)
182 (100%) 93 (100%) 44 (100%) 30 (100%) 3 (100%) 11 (100%) 363 (100%)
Movement Contribution In order to better understand the feature of interchange ramp terminal crashes, the movements for at fault vehicles (according to police report) were summarized and shown in Table 4. Except for going straight, which is the most frequent movement type, slowing down and stopped (39.0%) is the most frequent movement at fault for a rear-end crash; and left turning (35.5%) is the most frequent movement at fault for an angle crash. When left turning is at fault, approximately 50 percent (33 out of 64) of crashes were angle crashes; and 84.5 percent (71 out of 84) were rear-end crashes. There were 50 angle crashes with going straight at faults. Among these, drivers going straight disregarded traffic control in 23 crashes and failed to yield in another seven crashes. Additionally, in 30 of the 50 angle crashes, the other vehicle that was involved in the crash was making a left turn. There are 81 rear-end crashes with a going straight movement at fault. And in 53 out of 81 crashes, the other vehicle that was involved in the crash was slowing down or stopped.
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Logically, left turning can lead to angle crashes, and slowing down or stopped leads to rear-end crashes. Considering that rear-end and angle crashes share about 75 percent of total crashes at these ramp terminals, left turning and slowing down or stopped should have a substantial effect on crash frequency. As slowing down or stopped relates to clearance and change intervals, clearance and change intervals, i.e., yellow and all-red interval, can be inferred to have some impact on crash frequency. Table 5. Crash Type for Interchange Ramp Terminals with or without Off Ramp Exclusive Right Turn Phase. Rear-End Interchange Group Without off ramp exclusive right 105 turn phase (48.4%) (ID1,2,4,6)
Angle
Sideswipe
No Collision
Head On
Other
Total
49 (22.6%)
28 (22.5%)
26 (12.0%)
2 (0.9%)
7 (3.25)
217 (100%)
With off ramp exclusive right turn phase (ID3,5)
77 (52.7%)
44 (30.1%)
16 (11.0%)
4 (2.7%)
1 (0.7%)
4 (2.7%)
146 (100%)
All Sites
182 (50.1%)
93 (25.6%)
44 (12.1%)
30 (8.3%)
3 (0.8%)
11 (3.0%)
363 (100%)
Effect of Exclusive Right Phase Within the six interchanges, four interchanges have no off ramp with an exclusive right turn phase, one interchange (ID 5) has one off ramp with an exclusive right turn phase, and one interchange (ID 3) has an exclusive right turn phase for both off ramps. To show the effect of the off ramp exclusive right turn phase, six interchanges were divided into two groups: with and without an off ramp exclusive right turn phase. As shown in Table 5, the group with an off ramp had a higher percentage of rear-end crashes and a lower percentage of angle crashes. Investigating the movement for at fault vehicles, it is found that right tuning at fault crashes had a lower percentage at ramp terminals with an off ramp exclusive right turn phase (8.9%) than ramp terminals without (12.4%). Further, looking at these crashes with right turn vehicle at fault, it is found that ramp terminals with an off ramp exclusive right turn phase (0 angle and 8 rear-end out of 13 crashes) had a lower percentage of angle crashes and a higher percentage of rear-end crashes than ramps terminals without (14 angle and 3 rear-end out of 27 crashes). By considering different geometric conditionions for these two groups of interchanges, it can be inferred that off ramp exclusive right turn phases can transform some angle crashes near the off ramp into rear-end crashes which are less severe, as shown in Figure 5.
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Figure 5. Crash and conflict type at off ramps. Table 6. Crash Type for Interchange Ramp Terminals with or without Adequate Yellow Interval. Interchange Group Rear-End Without 154 adequate yellow (51.9%) interval (ID2,3,5,6) With 28 adequate yellow interval (42.4%) (ID1,4) 182 All Crashes (50.1%)
No Collision Head On
Angle
Sideswipe
Other
Total
82 (27.6%)
29 (9.8%)
19 (6.4%)
2 (0.7%)
11 (3.7%)
297 (100%)
11 (16.7%)
15 (22.7%)
11 (16.7%)
1 (1.5%)
0 (0%)
66 (100%)
93 (25.6%)
44 (12.1%)
30 (8.3%)
3 (0.8%)
11 (3.0%)
363 (100%)
Effect of Yellow Interval The yellow intervals at the six pairs of ramp terminals range between 3.5 s and 4.5 s. Compared with the ITE recommended yellow interval, four out of six sites had inadequate yellow intervals. Comparing the two group of sites (with or without adequate yellow intervals, as shown in Table 6), the group without adequate yellow intervals has a higher crash percentage in both rear-end and angle crashes than those with an adequate yellow interval (rear-end crash 51.9% vs. 42.4%; angle crash 27.6% vs. 16.7%). This implies that rear-end and angle crashes are influenced by the yellow interval. Considering that rear-end and angle crashes are the major two crash type, yellow intervals do have an impact on crashes at ramp terminals. MODEL DEVELOPING AND INTERPRETATION Generalized linear regression models were used to develop the crash frequency model. The developed model is shown as b Ecrash is the expected Ecrash a VE exp c Ydif d ARdif eRTdummy f Sterminal .
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crash frequency; and a,b,c,d,e,f are the coefficients or constants. Independent variables are introduced as following: (1) VE AADTramp1 AADTcrd 1 AADTramp 2 AADTcrd 2 : volume exposure, reflecting the actual effect of volume. AADTramp1 and AADTramp2 represent the AADT of two off ramps, AADTcrd1 and AADTcrd2 represent the AADT of crossroads at the two ends of ramp terminals. Va (2) Ydif Yobserved YITE Yobserved Tpr : difference between 2d r 2 gGr observed yellow intervals and the ITE recommended yellow intervals. dr is the deceleration rate (10 ft/s2); g is the gravitational acceleration and equal to 32.2 ft/s2; Gr is the approach grade (ft/ft); Tpr is the driver perception-reaction time and equal to 1.0 s; and Va is the speed of a vehicle approaching the intersection (ft/s) and here equal to the speed limit. SL : difference between observed all(3) ARdif ARobserved ARITE ARobserved Va red intervals and ITE recommended all-red intervals. S is the actual moving path length (ft) and here refers to left turn curve length; L is the vehicle length and equal to 20 ft; Va is the speed (ft/s) of the vehicle making movement and here assumes a left turning moving at speed limit. It should be noted that only the crossroad left turn all-red intervals are considered here, since preliminary analysis has shown that a left turn is the most dangerous movement. Also, off ramp left turns are in protected only, while crossroad left turns are in both protected and permissive. (4) RTdummy: the dummy variable shows whether or not an off ramp has an exclusive right turn phase. If any one of the two off ramps has exclusive right turn phase, the dummy variable will be 1, otherwise 0. (5) Sterminal: is the Spacing between two ramp terminals (ft). For total rear-end and angle crashes, three different models are developed. By using the stepwise regression approach, the model results were presented in Table 7. Table 7. Model Results for All Crashes, Rear-End Crashes and Angle Crashes. Variables Contant,(lna) ln(VE) Ydif RTdummy spacing ARdif R-sq R-sq(adj)
All Crashes Coefficient P-value -21.809 0.000 0.9386 0.000 -1.8419 0.007 -1.3205 0.000 0.014308 0.000 -0.988 0.000 0.893 0.871
Rear-End Crashes Coefficient P-value -15.129 0.000 0.6585 0.000 -2.3424 0.011 -0.9844 0.013 0.009803 0.001 -0.8944 0.012 0.810 0.769
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Angle Crashes Coefficient P-value -26.572 0.000 1.0067 0.000 -3.504 0.007 -2.078 0.001 0.01985 0.000 -1.8502 0.001 0.844 0.800
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The negative coefficient for the yellow interval difference indicates that the crash frequency will decrease as the difference in the yellow interval increases (when the observed yellow interval is longer than ITE recommended). Further, the crash frequency will increase as the difference in yellow interval increases (when the observed yellow interval is shorter than ITE recommended value). This result is the same as conventional intersections and has been proven by previous research (Zador et al., 1985; Bonneson et al., 2006; Zimmerman et al., 2005; Wang et al., 2007). Different coefficient values of yellow interval difference for total, rear-end, and angle crashes indicate that yellow interval difference has different effects on total, rear-end, and angle crash frequency. With everything else constant, based on the model in Table 7, increasing the yellow interval by 1.0 s will decrease about 84 percent (equal to e-1.8419-1) of total crashes. Previous research shows that for conventional intersections, a 1.0 s increase in the yellow interval will lead to a reduction of 40 percent of total crashes (Bonneson et al., 2006). This difference indicates that yellow intervals may have a more significant effect on interchange ramp terminals than conventional intersections. Furthermore, the absolute coefficient value for yellow intervals is larger than all-red intervals. This indicates that the marginal effect of the yellow interval is greater than the all-red interval. The negative coefficient of the all-red interval indicates that an all-red interval has a similar effect as the yellow interval. The crash frequency will decrease as the allred interval difference increases when the observed all-red interval is longer than the ITE recommended value. And it increases as the all-red interval difference increases when the observed all-red interval is shorter than the ITE recommended value. If all other situations remain constant, increasing the all-red interval by 1.0 s will lead to an approximately 62 percent (equal to e-0.988-1) reduction in total crashes. Additionally, as the absolute value of the all-red interval coefficient for an angle crash (1.8502) is larger than a rear-end crash (0.8944), the all-red interval has a larger marginal effect on an angle crash. The reasons for this different effect includes the following: (1) red light running has a larger probability of causing angle crashes than rear-end crashes; (2) allred intervals used in this model only include crossroad left turns, which are a major contributing movement for angle crashes. Considering an off ramp exclusive right turn phase, the negative coefficient indicates that ramp terminals having off ramps with an exclusive right turn phase will have smaller crash frequency than those without (if all other conditions keep constant). The larger absolute value of the coefficient for angle crashes indicates that an off ramp exclusive right turn phase has a larger effect on angle crash. This result is consistent with preliminary analysis. For the effect of terminal spacing, the positive coefficient indicates that as the terminal spacing between two terminals increases, the crash frequency increases. However, due to the low absolute value, the effect is not substantial. Different values of coefficients for rear-end crashes and angle crashes indicate the different effect of terminal spacing on these two types of crashes. With the terminal spacing increasing,
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two ramp terminals are more separated and more similar to two separated conventional intersections, having a higher percentage of angle crashes and a lower percentage of rear-end crashes. SUMMARY AND CONCLUSIONS A safety performance function for signalized diamond interchange ramp terminals was developed by using data collected from six diamond interchanges. The major factors considered include volume, clearance interval, off ramp exclusive right turn phase, and spacing between two terminals. After the analysis, the following conclusions have been researched: (1) Rear-end and angle crashes are the two major types of crashes at signalized diamond interchange ramp terminals. Left turns and change in speed, i.e., braking, are the most critical movements contributing to ramp terminal crashes. (2) The off ramp exclusive right turn phase, combined with its specific geometric conditions, can transform some kind of angle crash related conflicts into rear-end crash related conflicts. Thus, if the interchange ramp terminals have an off ramp exclusive right turn phase, the possibility of rear-end crash increases while the possibility of angle crash decreases. (3) For yellow and all-red intervals, the crash frequency will increase with the deficiency of yellow or all-red interval increases when comparing with recommended the yellow or all-red interval value calculated by the ITE equation. The generalized linear model results indicate that a 1.0 s increase in the yellow interval will decrease about 84 percent of total crashes, while that number for a 1.0 s increase in all-red intervasl is 62 percent. (4) With terminal spacing increasing, the crash frequency will increase; and for angle crash and rear-end crash, the safety effects of terminal spacing are different. For further research, more diamond interchanges should be analyzed to validate the conclusion received as 6 interchanges may not be enough. Also, SPFs for other type of interchange ramp terminals should be created. REFERENCES Bonneson, J.A., Son, H.J. (2003) “Prediction of Expected Red-Light-Running Frequency at Urban Intersections.” Transportation Research Record: Journal of the Transportation Research Board. No. 1830, pp. 38-47. Bonneson, J.A, Zimmerman, K.H. (2004) “Effect of Yellow-Interval Timing on the Frequency of Red-Light Violations at Urban Intersections.” Transportation Research Record: Journal of the Transportation Research Board. No. 1865, pp. 20-27. Bonneson, J.A., Zimmerman, K. (2006) “Identifying Intersections with Potential for Red Light-Related Safety Improvement.” Transportation Research Record: Journal of the Transportation Research Board. No. 1953, pp. 128-136. Gates, T.J., Noyce, D.A., Laracuente, L., Nordheim, E.V. (2007) “Analysis of Driver Behavior in Dilemma Zones at Signalized Intersections.” Transportation
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ICCTP 2010: Integrated Transportation Systems— Green•Intelligent•Reliable © 2010 ASCE
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