TFHRC Home > Safety > Safety Publications > Surrogate Safety Assessment Model and Validation: Final Report > Chapter 3 (Continued)
Various types of intersections have been implemented and evaluated in three simulation systems: VISSIM, TEXAS, and AIMSUN.
The goal of this portion of the validation effort was not to compare the results of the simulation model with traffic at a comparable real-world location. Hence, no calibration effort was necessary or performed in this study. Reasonable driver behavior was verified, and appropriate control measures were used to avoid gridlock during high-volume test cases. As such, for all the intersection designs, default driving behavior models and parameters were applied for each simulation model. The same underlying simulation parameters were used for each comparison case to maintain comparability.
Eleven comparison cases were executed among the three simulation systems as follows:
Three sets of traffic volumes (low, medium, and high) were applied for each intersection design, and timing plans were designed to ensure no over-saturation would occur.
There are two basic alternative designs for left turns: protected and permitted. Protected left-turn design allocates an exclusive phase for left turn only, which will make the left-turn maneuvers have fewer conflict events with the opposing through traffic. Permitted left-turn design allows vehicles to make a left turn during the through traffic green phase and provides no specific green phase for left-turn only. This logic applies mostly to traffic conditions with low left-turn volumes. When the left-turn volumes become higher, there have been more conflict events when drivers begin accepting smaller gaps to cross the intersection.
According to the crash prediction models for four-leg signalized intersection, the existence of left-turn phase will result in lower crash frequency. Thus, it is hypothesized that protected left turn should have lower predicted conflict frequency than permitted left turn when other network parameters remain the same. Also, it would be reasonable to expect that severity values of the surrogate measures would be less critical for protected left turns versus permitted left turns.
Intersection Description for Case Study
The intersection used to test the alternative traffic control logic for left turns is four-legged intersections with three through lanes in the main travel directions and two through lanes on the side-street approaches to the intersection, as shown in figure 29. All left-turn bays are 76.25 m (250 ft) long.
Figure 29. Screen Capture. Intersection Geometry for Testing Control Logic.
Table 8 lists the traffic volumes applied for each approach of the intersection. Fixed time traffic control is applied in this test. Figure 30 through figure 35 provide the key timing plan parameters for each testing scenario.
Approach | Southbound | Northbound | Eastbound | Westbound | ||||||||
L | TH | R | L | TH | R | L | TH | R | L | TH | R | |
Phase# (Permitted) |
4 | 4 | 8 | 8 | 2 | 2 | 6 | 6 | ||||
Phase# (Protected) |
7 | 4 | 3 | 8 | 5 | 2 | 1 | 6 | ||||
Low Volume | 100 | 350 | 50 | 100 | 350 | 50 | 60 | 210 | 30 | 60 | 210 | 30 |
Medium Volume | 240 | 400 | 160 | 240 | 400 | 160 | 180 | 240 | 180 | 180 | 240 | 180 |
High Volume | 300 | 1050 | 150 | 300 | 1,050 | 150 | 240 | 840 | 120 | 240 | 840 | 120 |
Note: L, TH, and R correspond to vehicles proceeding left, through, or right at the intersection.
Figure 30. Illustration. Timing Plan for Permitted Left Turn in Low Volumes.
Figure 31. Illustration. Timing Plan for Protected Left Turn in Low Volumes.
Figure 32. Illustration. Timing Plan for Permitted Left Turn in Medium Volumes.
Figure 33. Illustration. Timing Plan for Protected Left Turn in Medium Volumes.
Figure 34. Illustration. Timing Plan for Permitted Left Turn in High Volumes.
Figure 35. Illustration. Timing Plan for Protected Left Turn in High Volumes.
Data Analysis and Comparison Results
Ten replications were performed for each design case and the resulting output trajectory data were analyzed by SSAM. F-test and t-tests were applied to compare surrogate measures of safety and the aggregations of those measures.
Table 9 through Table 13 list the values of all surrogate measures of safety and corresponding t test results for different types of aggregations with the low-speed events and crash data excluded (TTC ≤ 0 and MaxS ≥ 16.1 km/h (10 mi/h)).
Total |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
PER | PRO | PER | PRO | PER | PRO | |
Low volume Mean |
4.7 |
4.7 |
16.1 |
14 |
25.7 |
22.4 |
Variance |
6.2 |
5.3 |
11.2 |
11.3 |
12.0 |
17.2 |
t-value(95%), difference (%) |
0 |
1.399 |
1.932 |
|||
Medium volume Mean |
13.7 |
12.8 |
33.1 |
32.6 |
53.7 |
63.3 |
Variance |
21.6 |
8.2 |
21.4 |
24.7 |
39.8 |
113.8 |
t-value(95%), difference (%) |
-0.522 |
0.233 |
-2.45, -17.88% | |||
High volume Mean |
108.3 |
174.2 |
184.1 |
489.1 |
309.5 |
1208.6 |
Variance |
138.7 |
183.1 |
332.8 |
189.2 |
408.1 |
1344.5 |
t-value(95%), difference (%) |
-11.618 , -60.85% |
-42.216, -165.67% |
-67.916, -290.5% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table illustrates a counterintuitive result. The design with the protected left turn has, on average, more total conflicts than the case with the permitted left turn for medium- and high-traffic volumes. The following tables explain this result by breaking the total results into a result for each conflict type.
Crossing |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
PER | PRO | PER | PRO | PER | PRO | |
Low volume Mean |
3.1 |
0.3 |
5.2 |
0.4 |
6.3 |
1 |
Variance |
3.4 |
0.5 |
4.4 |
0.5 |
4.2 |
1.1 |
t-value(95%), difference (%) |
4.490, 90.32% |
6.865, 92.31% |
7.250, 84.13% |
|||
Medium volume Mean |
5.2 |
0.6 |
8.1 |
0.8 |
10.1 |
1.8 |
Variance |
7.3 |
0.7 |
5.0 |
0.8 |
10.1 |
0.8 |
t-value(95%), difference (%) |
5.143, 88.46% |
9.558, 90.12% |
8.89, 92.808 |
|||
High volume Mean |
23.3 |
6.4 |
27.5 |
9.2 |
33.8 |
15 |
Variance |
46.9 |
6.7 |
67.2 |
11.1 |
71.5 |
16.0 |
t-value(95%), difference (%) |
7.299, 72.53% |
6.543, 66.75% |
6.355, 55.62% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table indicates what is expected to happen from adding a protected left-turn phase; that the total crossing conflicts are reduced for all levels of traffic volume. This indicates that SSAM, in its most basic form, is a valid indicator of safety.
Rear End |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
PER | PRO | PER | PRO | PER | PRO | |
Low volume Mean |
1.3 |
3.9 |
8.6 |
11.4 |
15.4 |
17.7 |
Variance |
0.7 |
2.3 |
12.5 |
7.6 |
21.2 |
10.0 |
t-value(95%), difference (%) |
-4.747, -200.00% | -1.976 |
-1.303 |
|||
Medium volume Mean |
5.3 |
5.2 |
17.2 |
19.5 |
29.7 |
42.9 |
Variance |
7.6 |
7.7 |
13.7 |
10.9 |
32.0 |
83.9 |
t-value(95%), difference (%) |
0.081 |
-1.464 |
-3.878, -44.44% | |||
High volume Mean |
19.3 |
84.5 |
56.3 |
309.1 |
122.9 |
848.4 |
Variance |
34.7 |
174.1 |
125.8 |
109.9 |
202.8 |
957.4 |
t-value(95%), difference (%) |
-14.271 , -337.82% |
-52.075, -449.02% |
-67.357 , -590.32% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table indicates the large increase in rear-end conflicts for high and medium volumes that is generated by adding the protected left-turn phase. This large increase in rear-end events is the primary cause of the total conflicts being counter-indicative. Thus, it should be important to analyze all types of conflicts rather than just examining the total number of events when comparing designs.
LC |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
PER | PRO | PER | PRO | PER | PRO | |
Low volume Mean |
0.3 |
0.5 |
2.3 |
2.2 |
4 |
3.7 |
Variance |
0.5 |
0.9 |
2.0 |
4.0 |
2.4 |
4.5 |
t-value(95%), difference (%) |
-0.535 |
0.129 |
0.361 |
|||
Medium volume Mean |
3.2 |
7 |
7.8 |
12.3 |
13.9 |
19.6 |
Variance |
1.5 |
7.6 |
8.0 |
24.7 |
9.2 |
40.7 |
t-value(95%), difference (%) |
-3.991 , -118.75% |
-2.491 , -57.69% |
-2.551, -41.01% |
|||
High volume Mean |
65.7 |
83.3 |
100.3 |
170.8 |
152.8 |
345.2 |
Variance |
101.1 |
26.2 |
142.0 |
148.0 |
93.3 |
381.1 |
t-value(95%), difference (%) |
-4.932,-26.79% |
-13.092,-70.29% |
-27.935,-125.92% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table indicates a 40-percent increase in lane change events at medium volumes and a 125-percent increase at high volumes. These results are likely due to longer queues in the left-turn bay because there is no permitted portion of the phase.
TPER | TPRO | CPER | CPRO | REPER | REPRO | LPER | LPRO | |
---|---|---|---|---|---|---|---|---|
TTC (low) |
0.92 |
0.91 |
0.6 |
0.99 |
1.03 |
0.9 |
1.01 |
0.92 |
t- value, diff(%) |
0.299 |
-2.547, 65.00% | 3.849, 12.62% | 1.112 |
||||
TTC (med) |
0.89 |
0.97 |
0.59 |
0.29 |
0.97 |
1.06 |
0.94 |
0.81 |
t -value, diff(%) |
-3.167, -8.99% | 1.776 |
-3.256, -9.28% | 2.639, 13.83% | ||||
TTC (high) |
0.8 |
1.05 |
0.46 |
0.74 |
1.01 |
1.11 |
0.71 |
0.92 |
t -value, diff(%) |
-25.245, -31.25% |
-5.922, 60.87% |
-8.067, -9.90% |
-13.285, -29.58% |
||||
PET(low) |
2.21 |
1.83 |
0.92 |
1.75 |
2.63 |
1.86 |
2.64 |
1.71 |
t -value, diff(%) |
3.796, 17.19% | -1.991 |
7.580, 29.28% |
3.912, 35.23% |
||||
PET(med) |
1.81 |
1.85 |
0.92 |
0.15 |
2.13 |
2.2 |
1.75 |
1.17 |
t -value, diff(%) |
-0.532 |
8.840, 83.70% | -0.769 |
4.046, 33.14% | ||||
PET(high) |
1.42 |
2.08 |
0.58 |
0.73 |
2.07 |
2.41 |
1.08 |
1.33 |
t -value, diff(%) |
-26.886, -46.48% |
-2.218, 25.86% |
-8.598, -16.43% |
-8.445, -23.15% |
||||
MaxS(low) |
26.7 |
33.22 |
34.5 |
38.64 |
24.29 |
33.3 |
23.69 |
31.35 |
t -value, diff(%) |
-7.947, -24.42% |
-3.091, 12.00% |
-9.521, -37.09% |
-3.601, -32.33% |
||||
MaxS(med) |
32.4 |
27.94 |
37.41 |
37.73 |
30.56 |
26.37 |
32.68 |
30.98 |
t -value, diff(%) |
8.321, 13.77% | -0.159 |
5.916, 13.71% | 1.831 |
||||
MaxS(high) |
28.27 |
25.11 |
32.2 |
23.42 |
26.32 |
24.85 |
28.97 |
25.82 |
t -value, diff(%) |
20.782, 11.18% |
14.825, 27.27% |
6.757, 5.59% |
13.492, 10.87% |
||||
DeltaS(low) |
29.16 |
31.83 |
47.48 |
37.92 |
23.66 |
32.44 |
21.51 |
27.25 |
t -value, diff(%) |
-2.524, -9.16% | 5.886, 20.13% | -8.739, -37.11% |
-2.838, -26.69% |
||||
DeltaS(med) |
31.07 |
19.54 |
48.5 |
37.26 |
27.06 |
18.46 |
27 |
21.2 |
t -value, diff(%) |
13.651, 37.11% |
3.089, 23.18% |
8.425, 31.78% |
4.461, 21.48% |
||||
DeltaS(high) |
22.22 |
16.12 |
34.98 |
21.3 |
20.44 |
16.97 |
20.83 |
13.79 |
t -value, diff(%) |
27.963, 27.45% |
16.672, 9.11% |
11.610, 16.98% |
23.063, 33.80% |
||||
DR(low) |
-5.54 |
-6.91 |
-0.91 |
-7.22 |
-7.12 |
-6.96 |
-6.73 |
-6.57 |
t -value, diff(%) |
4.632, -24.73% |
6.826, 93.41% |
-0.555 |
-0.245 |
||||
DR(med) |
-5.12 |
-3.72 |
-1.7 |
-1.43 |
-6.15 |
-4.21 |
-5.4 |
-2.73 |
t -value, diff(%) |
-6.947, 27.34% | -0.235 |
-8.651, 31.54% |
-6.712, 49.44% |
||||
DR(high) |
-2.97 |
-4.77 |
-0.88 |
-0.56 |
-4.93 |
-5.61 |
-1.85 |
-2.88 |
t -value, diff(%) |
23.945, -60.61% |
-1.309 |
5.818, -13.79% |
10.026, -55.68% |
||||
MaxD(low) |
-11 |
-15.98 |
-2.09 |
-17.26 |
-14.03 |
-16.45 |
-13.37 |
-13.39 |
t -value, diff(%) |
9.844, -45.27% |
16.899, 25.84% |
5.388, -17.25% |
0.019 |
||||
MaxD(med) |
-13.04 |
-11.43 |
-4.28 |
-1.62 |
-15.25 |
-13.11 |
-14.7 |
-8.14 |
t -value, diff(%) |
-4.499, 12.35% |
-2.490, 62.15% |
-7.662, 14.03% |
-9.440 44.63% |
||||
MaxD(high) |
-7.97 |
-12.06 |
-2.02 |
-3.44 |
-12.97 |
-13.96 |
-5.27 |
-7.76 |
t -value, diff(%) |
31.265, -51.32% |
2.918, -70.30% |
7.012, -7.63% |
13.251, -47.25% |
||||
MaxDeltaV(low) |
16.91 |
18 |
28.86 |
20.85 |
13.21 |
18.35 |
12.36 |
15.56 |
t -value, diff(%) |
-1.594 |
5.112, 27.75% | -8.851, -38.91% |
-2.636, -25.89% |
||||
MaxDeltaV(med) |
18.02 |
11.2 |
29.22 |
21.68 |
15.5 |
10.47 |
15.24 |
12.38 |
t -value, diff(%) |
12.935, 37.85% |
2.312, 25.80% |
8.262, 32.45% |
3.647, 18.77% |
||||
MaxDeltaV(high) |
12.67 |
9 |
20.35 |
11.82 |
11.67 |
9.44 |
11.78 |
7.79 |
t -value, diff(%) |
27.676, 28.97% |
15.233, 41.92% |
12.307, 19.11% |
22.270, 33.87% |
Note: Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
Correlations with Predicted Crash Frequency
The predicted crash rates (crashes per year) for all scenarios in this test are listed in table 14 with the corresponding surrogate measures of safety (conflicts per hour). Rank orders for each category of data are also listed in the table. The Spearman rank correlation coefficients are calculated for each test.
AADT | Low | Medium | High | Rs | ||||
---|---|---|---|---|---|---|---|---|
PER |
PRO |
PER |
PRO |
PER |
PRO |
|||
Crash Frequency |
M |
5 |
3.3 |
7.5 |
5 |
12.6 |
8.4 |
1 |
R |
2 |
1 |
4 |
2 |
6 |
5 |
||
Total Conflict |
M |
25.7 |
22.4 |
53.7 |
63.3 |
309.5 |
1,208.6 |
0.77 |
R |
1 |
1 |
3 |
4 |
5 |
6 |
||
Crossing Conflict |
M |
6.3 |
1 |
10.1 |
1.8 |
33.8 |
15 |
0.89 |
R |
3 |
1 |
4 |
2 |
6 |
5 |
||
Rear-End Conflict |
M |
15.4 |
17.7 |
29.7 |
42.9 |
122.9 |
848.4 |
0.74 |
R |
1 |
1 |
3 |
4 |
5 |
6 |
||
LC Conflict |
M |
4 |
3.7 |
13.9 |
19.6 |
152.8 |
345.2 |
0.74 |
R |
1 |
1 |
3 |
4 |
5 |
5 |
Note: Rows labeled "M" provide mean values and rows labeled "R" provide the ranking of each alternative. The Rs column provides Spearman rank correlation coefficients indicating agreement with theoretical crash estimates.
Findings and Conclusions
Based on the observation of the safety surrogate test data, the following conclusions can be drawn:
In general, the surrogate measures present mixed results; however, an appropriate conclusion can be drawn with consideration of the differing severities of different conflict types. The addition of a protected left-turn phase tended to increase the total number of conflicts while the protected phase substantially decreased crossing conflicts, as expected. The increase in conflicts came primarily from rear ends and, under higher flows, from lane-changing maneuvers. This result is elucidated by considering that, in adding a protected left-turn phase, the cycle time was increased and the proportion of green time for through phases was decreased. This change in the timing has the effect of increasing the number of vehicle stops for through traffic. The number of vehicle stops is known to correlate with the number of rear-end crashes and thus with higher rear-end conflicts. It is also possible that with a greater proportion of vehicles arriving to a standing or dispersing queue, there is an increased tendency of drivers to change to a lane with a shorter queue, despite all lanes having stopped traffic. Drivers in free-flowing lanes may be relatively content to stay in their lane when all lanes are flowing at the same (nonzero) speed. Thus, the conflict frequency results appear reasonable and have provoked consideration of the effect of timing changes and driver behavior. However, the severity-related surrogate measures indicate that the protected left-turn case has improved average values (increased TTC and PET and decreased DeltaV, especially at high volumes), indicating that the protected left-turn phasing is safer than the permitted left-turn case, as would be expected.
The Spearman-rank correlation coefficients from all tests show a strong positive relationship between the rank orders of the surrogate measures of safety and the rank orders of the predicted crash rates. The relationships between the rank order of the totals of all conflict types and crossing conflict types are stronger than the relationship of rear-end and lane-change crossing conflicts. This, again, would be expected because it has been validated in the field that protected left turns reduce crossing crashes. TEXAS, however, shows a very high rate of rear-end and lane-change events per hour, indicating that the default driver behavior parameters may allow vehicles to perform maneuvers that allow closer proximity than the "rule of thumb" threshold of TTC = 1.5 would preclude in the real world.
A left-turn bay on an approach to an intersection provides an independent lane for the storage and movement of the left-turn vehicles. With the left-turn bay, the conflict events between through movement vehicles (primarily traveling in the same direction as the turn vehicles) and left-turn vehicles is hypothesized to be significantly reduced. This has been tested over a range of traffic volume scenarios from light traffic to heavy traffic.
According to the crash prediction models for all conventional intersections, the existence of a left-turn bay will reduce the crash frequency under the same traffic conditions.(13)
Intersection Description
The intersection used to test the left-turns bay versus no left-turn bay is a four-legged intersection with two through lanes with shared right turn for all approaches to the intersection, as shown in figure 36 and figure 37. All left-turn bays are 76.25 m (250 ft) long. Table 15 indicates the traffic volumes arriving to each approach of the intersection. Fixed-time traffic control is applied in this test. The ring-diagrams from figure 38 through figure 43 show the timing plans for each testing scenario.
Figure 36. Screen Capture. Exclusive Left-Turn Lane.
Figure 37. Screen Capture. Shared Use Left-Turn and Through Lane.
Approach | Southbound | Northbound | Eastbound | Westbound | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
L | TH | R | L | TH | R | L | TH | R | L | TH | R | |
Phase# (Permitted) |
4 | 4 | 8 | 8 | 2 | 2 | 6 | 6 | ||||
Low Volumes | 125 | 250 | 125 | 125 | 250 | 125 | 125 | 250 | 125 | 125 | 250 | 125 |
Medium Volumes | 200 | 400 | 200 | 200 | 400 | 200 | 200 | 400 | 200 | 200 | 400 | 200 |
High Volumes | 300 | 600 | 300 | 300 | 600 | 300 | 300 | 600 | 300 | 300 | 600 | 300 |
Note: L, TH, and R correspond to vehicles proceeding left, through, or right at the intersection.
Figure 38. Illustration. Timing Plan for Intersection with Left-Turn Bay in Low Volumes.
Figure 39. Illustration. Timing Plan for Intersection without Left-Turn Bay in Low Volumes.
Figure 40. Illustration. Timing Plan for Intersection with Left-Turn Bay in Medium Volumes.
Figure 41. Illustration. Timing Plan for Intersection without Left-Turn Bay in Medium Volumes.
Figure 42. Illustration. Timing Plan for Intersection with Left-Turn Bay in High Volumes.
Figure 43. Illustration. Timing Plan for Intersection without Left-Turn Bay in High Volumes.
Data Analysis and Comparison Results
Ten replications were performed for each simulation scenario, and the resulting output trajectory data were analyzed by SSAM. F-test and t-tests were applied to identify statistical significance. Table 16 through table 20 list the values of all surrogate measures of safety and corresponding t 'test results for different types of aggregations with the low speed events and crash data excluded (TTC ≤ 0 and MaxS ≥ 16.1 km/h (10 mi/h)).
Total |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NLB | WLB | NLB | WLB | NLB | WLB | |
Low volume Mean |
9.6 |
11.3 |
9.6 |
26.3 |
54 |
42.6 |
Variance |
7.2 |
26.9 |
7.2 |
45.8 |
68.7 |
63.8 |
tvalue(95%), difference (%) |
-0.921 |
-7.258, 173.96% | 3.132, 21.11% | |||
Medium volume Mean |
15.8 |
19.7 |
58.2 |
47.6 |
210.7 |
98.5 |
Variance |
18.8 |
19.3 |
166.2 |
32.3 |
704.5 |
53.6 |
t-value(95%), difference (%) |
-1.996 |
2.38, 18.21% |
12.887, 53.25% |
|||
High volume Mean |
140.8 |
150.1 |
506.8 |
279.1 |
985.9 |
487 |
Variance |
156.4 |
78.5 |
250.4 |
81.4 |
467.9 |
293.6 |
t-value(95%), difference (%) |
-1.919 |
39.528, 44.93% |
57.174, 50.6% |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay. Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
Crossing |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NLB | WLB | NLB | WLB | NLB | WLB | |
Low volume Mean |
5.3 | 8.5 | 5.3 | 12.3 | 11.9 | 15.3 |
Variance |
2.7 | 12.5 | 2.7 | 15.3 | 6.3 | 17.3 |
t-value(95%), difference (%) |
-2.597, -60.38% | -5.214, -132.08% | -2.210, -28.57% | |||
Medium volume Mean |
6.1 | 13.4 | 9 | 18.6 | 11 | 27.1 |
Variance |
5.9 | 6.9 | 7.8 | 13.4 | 7.1 | 23.2 |
t-value(95%), difference (%) |
-6.45, -119.67% | -6.6, -106.67% | -9.246, -146.36% | |||
High volume Mean |
9.1 | 34 | 15 | 45.6 | 19.4 | 60.8 |
Variance |
11.7 | 52.7 | 10.7 | 62.9 | 15.6 | 69.5 |
t-value(95%), difference (%) |
-9.818, -273.63% | -11.279, -204.00% | -14.191, -213.4% |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay. Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
This table indicates that, for all traffic volumes, the number of severe-crossing conflicts is increased when the left-turn bay is added. This result likely reflects the increase in the number of available left-turn maneuvers due to the bay.
Rear End |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NLB | WLB | NLB | WLB | NLB | WLB | |
Low volume Mean |
2.7 |
2.3 |
2.7 |
11.6 |
31.8 |
21.3 |
Variance |
3.1 |
3.8 |
3.1 |
18.3 |
40.2 |
42.7 |
t-value(95%), difference (%) |
0.481 |
-6.085, -329.60% | 3.648, 33% | |||
Medium volume Mean |
5.6 |
4 |
38.4 |
22.2 |
175.5 |
54.8 |
Variance |
10.933 |
6.222 |
129.378 |
17.956 |
637.611 |
23.067 |
t-value(95%), difference (%) |
1.222 |
4.221, 42.19% |
14.85, 68.77% |
|||
High volume Mean |
110.4 |
24.2 |
434.1 |
94.8 |
855.8 |
223 |
Variance |
131.8 |
44.6 |
209.0 |
183.7 |
377.3 |
194.7 |
t-value(95%), difference (%) |
20.521, 78.08% |
54.143, 78.16% |
83.673, 73.97% |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay. Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
This table indicates a definite decrease in the number of rear-end conflicts when a left- turn bay is added to the intersection, as expected from field experience.
Lane Change |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NLB | WLB | NLB | WLB | NLB | WLB | |
Low volume Mean |
1.6 |
0.5 |
1.6 |
2.4 |
10.3 |
6 |
Variance |
1.4 |
0.3 |
1.4 |
3.6 |
6.0 |
6.4 |
t-value(95%), difference (%) |
2.703, 68.80% | -1.134 | 3.853, 41.75% | |||
Medium volume Mean |
4.1 |
2.3 |
10.8 |
6.8 |
24.2 |
16.6 |
Variance |
1.878 |
3.567 |
4.844 |
8.622 |
31.289 |
27.156 |
t-value(95%), difference (%) |
2.439, 43.9% |
3.447, 37.04% |
3.144, 31.4% |
|||
High volume Mean |
21.3 |
91.9 |
57.7 |
138.7 |
110.7 |
203.2 |
Variance |
21.6 |
93.7 |
53.3 |
177.3 |
113.1 |
242.6 |
t-value(95%), difference (%) |
-20.799, -331.46% |
-16.864, -140.38% |
-15.509, -83.56% |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay. Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
TNLB | TWLB | CNLB | CWLB | RENLB | REWLB | LCNLB | LCWLB | |
---|---|---|---|---|---|---|---|---|
TTC (low) |
0.97 |
0.86 |
0.67 |
0.56 |
1.08 |
1.01 |
1.01 |
1.09 |
t -value, diff(%) |
3.928, 11.34% | 1.804 |
2.428, 6.48% | 1.339 |
||||
TTC (med) |
1.17 |
0.97 |
0.54 |
0.66 |
1.23 |
1.11 |
1.02 |
1.04 |
t -value, diff(%) |
12.404, 17.09% | -2.048, -22.22% | 7.646, 9.76% |
-0.506 |
||||
TTC (high) |
0.99 |
0.84 |
0.64 |
0.58 |
1 |
1.07 |
0.96 |
0.68 |
t -value, diff(%) |
18.245, 15.15% | 1.484 |
-7.507,-7.00% | 16.155, 29.17% | ||||
PET(low) |
2.22 |
2.07 |
1.44 |
1.29 |
2.56 |
2.5 |
2.07 |
2.51 |
t -value, diff(%) |
1.732 |
0.880 |
0.670 |
-1.866 |
||||
PET(med) |
2.75 |
2.27 |
1.26 |
1.47 |
3.03 |
2.71 |
1.45 |
2.11 |
t -value, diff(%) |
9.464, 17.45% | -1.364 |
5.996, 10.56% | -5.236, -45.52% | ||||
PET(high) |
1.77 |
1.64 |
1.16 |
0.95 |
1.82 |
2.38 |
1.49 |
1.02 |
t -value, diff(%) |
5.860, 7.34% |
2.252, 18.10% |
-18.904, -30.77% | 12.073, 31.54% | ||||
MaxS(low) |
30.75 |
29.65 |
33.6 |
34.07 |
29.54 |
27.06 |
31.18 |
27.6 |
t -value, diff(%) |
1.833 |
-0.518 |
2.933, 8.40% |
2.353, 11.48% |
||||
MaxS(med) |
27.01 |
29.95 |
34.33 |
34.77 |
26.16 |
27.11 |
29.88 |
31.45 |
t -value, diff(%) |
-8.429, -10.88% | -0.499 |
-2.248, -3.63% | -1.623 |
||||
MaxS(high) |
24.38 |
28.78 |
28.44 |
31.78 |
24.12 |
27.38 |
25.64 |
29.42 |
t -value, diff(%) |
-31.808, -18.05% |
-5.551, -11.74% |
-17.846, -13.52% |
-12.115, -14.74% |
||||
DeltaS(low) |
28.57 |
30.05 |
38.86 |
39.96 |
25.03 |
24.93 |
27.59 |
22.95 |
t -value, diff(%) |
-1.788 |
-0.732 |
0.104 |
2.848, 16.82% | ||||
DeltaS(med) |
19.23 |
28.28 |
38.02 |
41.46 |
18.05 |
23.46 |
19.21 |
22.69 |
t -value, diff(%) |
-18.607, -47.06% |
-2.558, -9.05% |
-11.112, -29.97% |
-3.351, -18.12% |
||||
DeltaS(high) |
18.97 |
23.24 |
32.31 |
34.25 |
18.74 |
21.36 |
18.45 |
22.02 |
t -value, diff(%) |
-23.467, -22.51% |
-2.555, -6.00% |
-11.461, -13.98% |
-10.732, -19.35% |
||||
DR(low) |
-6.3 |
-5.32 |
-2.48 |
-1.47 |
-7.48 |
-7.49 |
-7.08 |
-7.43 |
t -value, diff(%) |
-3.323, 15.56% |
-2.022, 40.73% |
0.032 |
0.476 |
||||
DR(med) |
-6.1 |
-5.65 |
-0.49 |
-1.57 |
-6.6 |
-7.55 |
-5.04 |
-6.02 |
t -value, diff(%) |
-2.539, 7.38% | 3.233, -220.41% |
4.818, -14.39% |
2.373, -19.44% |
||||
DR(high) |
-4.79 |
-3.83 |
-0.03 |
-1.2 |
-5.1 |
-6.19 |
-3.27 |
-2.02 |
t -value, diff(%) |
-13.525, 20.04% | 8.067, -3900.00% |
11.276, -21.37% |
-9.530, 38.23% | ||||
MaxD(low) |
-12.86 |
-11.36 |
-5.63 |
-5.52 |
-15.11 |
-14.95 |
-14.26 |
-13.52 |
t -value, diff(%) |
-3.487, 11.66% | -0.135 |
-0.419 |
-0.838 |
||||
MaxD(med) |
-13.64 |
-11.91 |
-2.09 |
-5.21 |
-14.62 |
-14.86 |
-11.78 |
-13.13 |
t -value, diff(%) |
-7.813, 12.68% | 5.299, -149.28% | 1.371 |
2.427, -11.46% | ||||
MaxD(high) |
-13.56 |
-9.09 |
-1.98 |
-2.72 |
-14.11 |
-14.15 |
-11.34 |
-5.45 |
t -value, diff(%) |
-41.995, 32.96% | 2.288, -37.37% | 0.398 |
-30.409, 51.94% | ||||
MaxDeltaV(low) |
16.35 |
17.23 |
22.74 |
23.05 |
14.27 |
14.21 |
15.39 |
13.08 |
t -value, diff(%) |
-1.706 |
-0.309 |
0.103 |
2.448, 15.01% | ||||
MaxDeltaV(med) |
10.87 |
16.14 |
23.86 |
24.42 |
10.06 |
13.09 |
10.84 |
12.72 |
t -value, diff(%) |
-17.353, -48.48% | -0.586 |
-10.731, -30.12% |
-3.140, -17.34% |
||||
MaxDeltaV(high) |
10.62 |
13.23 |
19.46 |
19.61 |
10.44 |
12.05 |
10.44 |
12.61 |
t -value, diff(%) |
-24.135, -24.58% | -0.286 |
-11.999, -15.42% |
-10.832, -20.79% |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay, and these abbreviations are prepended with T-, C-, RE-, and LC- to indicate data based on total, crossing, rear-end, and lane-change conflicts respectively. Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
In general, the data in the table 20 have some counter-indicative results. Some of the average surrogate measures of safety are better with the left-turn bay, and others are worse.
Correlations with Predicted Crash Frequency
The predicted crash rates (crashes per year) for all scenarios in this test are listed in table 21 with the corresponding average conflicts per hour. Rank orders for each category of data are also listed in the table. The Spearman rank correlation coefficients are calculated for each test.
AADT | Low |
Medium |
High |
Rs | ||||
---|---|---|---|---|---|---|---|---|
NLB | WLB | NLB | WLB | NLB | WLB | |||
Crash Frequency | M | 5.5 |
3.7 |
8 |
5.3 |
8.7 |
5.8 |
1 |
R | 3 |
1 |
5 |
2 |
6 |
4 |
||
Total Conflict | M | 54 |
42.6 |
210.7 |
98.5 |
985.9 |
487 |
0.8 |
R | 1 |
1 |
4 |
3 |
6 |
5 |
||
Crossing Conflict | M | 11.9 |
15.3 |
11 |
27.1 |
19.4 |
60.8 |
0 |
R | 2 |
3 |
1 |
5 |
4 |
6 |
||
Rear-End Conflict | M | 31.8 |
21.3 |
175.5 |
54.8 |
855.8 |
223 |
0.8 |
R | 1 |
1 |
4 |
3 |
6 |
5 |
||
LC Conflict | M | 10.3 |
6 |
24.2 |
16.6 |
110.7 |
203.2 |
0.6 |
R | 2 |
1 |
3 |
4 |
5 |
6 |
Note: NLB indicates no left-turn bay and WLB indicates with left-turn bay. Rows labeled "M" provide mean values and rows labeled "R" provide the ranking of each alternative. The Rs column provides Spearman rank correlation coefficients indicating agreement with theoretical crash estimates.
Findings and Conclusions
Based on the observation on the total number of conflicts of various types and the average values of the surrogate measures obtained from the test, the following conclusions can be drawn:
In general, an intersection with a left-turn bay experiences fewer total and rear-end conflicts but more crossing and lane-change conflicts than an intersection without a left- turn bay. Rear-end conflicts constitute a major part of total conflicts (ranging from 60 to 80 percent) and have larger TTC and PET values (≥ 1.0).
The Spearman rank correlation coefficients resulted from all tests show a strong positive relationship between the rank orders of the surrogate measures of safety and the rank orders of the predicted crash rates, except for crossing conflicts, which shows no correlation with total crash rates. Perhaps a more normative comparison could be made by using rates of conflict occurrence by maneuver rather than total number of conflicts without relation to the number of other maneuvers that were executed by drivers without a conflict occurring.
TEXAS shows a very high rate of rear-end and lane-change events per hour, indicating that the default driver behavior parameters may allow vehicles to perform maneuvers that allow closer proximity than the "rule of thumb" threshold of TTC = 1.5 would preclude in the real world. Also, the existence of the turn bay requires more lane-changing maneuvers and thus a higher frequency of conflict events related to those necessary lane changes.
A right-turn bay near the intersection provides an independent lane for the storage and movement of right-turn vehicles. With a right-turn bay near an intersection, the conflict events between through-movement vehicles (primarily traveling in the same direction as the turn vehicles) and right-turn vehicles is hypothesized to be reduced significantly. This reduction has been tested over a range of traffic volume scenarios, from light traffic to heavy traffic.
According to the crash prediction models for all conventional intersections, the existence of a right-turn bay will definitely reduce the crash frequency when all other roadway network factors remain the same.(13)
Intersection Description
The intersection used to test the right-turn bay versus no right-turn bay is a four-legged intersection with two through lanes and one left-turn lane for all approaches to the intersection, as shown in figure 44 and figure 45. All left-turn bays have are 76.25-meters (250-feet) long. Table 22 shows the traffic volumes applied for each approach of the intersection. Fixed time traffic control is applied in this test. Figure 46 through figure 51 provide the timing plans for each testing scenario.
Figure 44. Screen Capture. Intersection with Right-Turn Bay.
Figure 45. Screen Capture. Intersection without Right-Turn Bay.
Approach | Southbound |
Northbound |
Eastbound |
Westbound |
||||||||
L |
TH |
R |
L |
TH |
R |
L |
TH |
R |
L |
TH |
R |
|
Phase ID | 7 |
4 |
3 |
8 |
5 |
2 |
1 |
6 |
||||
Low Volumes | 125 |
250 |
125 |
125 |
250 |
125 |
125 |
250 |
125 |
125 |
250 |
125 |
Medium Volumes | 200 |
400 |
200 |
200 |
400 |
200 |
200 |
400 |
200 |
200 |
400 |
200 |
High Volumes | 300 |
600 |
300 |
300 |
600 |
300 |
300 |
600 |
300 |
300 |
600 |
300 |
Note: L, TH, and R correspond to vehicles proceeding left, through, or right at the intersection.
Figure 46. Illustration. Timing Plan for Intersection with Right-Turn Bay in Low Volumes.
Figure 47. Illustration. Timing Plan for Intersection without Right-Turn Bay in Low Volumes.
Figure 48. Illustration. Timing Plan for Intersection with Right-Turn Bay in Medium Volumes.
Figure 49. Illustration. Timing Plan for Intersection without Right-Turn Bay in Medium Volumes.
Figure 50. Illustration. Timing Plan for Intersection with Right-Turn Bay in High Volumes.
Figure 51. Illustration. Timing Plan for Intersection without Right-Turn Bay in High Volumes.
Data Analysis and Comparison Results
Ten replications were performed for each simulation scenario, and the resulting output trajectory data was analyzed by SSAM. The F-test and t-test were applied to compare surrogate measures of safety and the aggregations of those measures. Table 23 through table 27 provide the values of all surrogate measures of safety and corresponding t-test results for different types of aggregations with the low speed events and crash data excluded (TTC ≤ 0 and MaxS ≥ 16.1 km/h (10 mi/h)).
Total |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NRB | WRB | NRB | WRB | NRB | WRB | |
Low volume Mean |
5.8 |
3.5 |
16.4 |
14.7 |
35.9 |
24.2 |
Variance |
7.067 |
3.611 |
17.822 |
9.122 |
38.544 |
18.844 |
t-value(95%), difference (%) |
2.226, 39.66% | 1.036 |
4.884, 32.59% | |||
Medium volume Mean |
12 |
10.1 |
43.4 |
31.2 |
119 |
67.3 |
Variance |
16.444 |
14.322 |
132.711 |
66.400 |
361.556 |
256.900 |
t-value(95%), difference (%) |
1.083 |
2.734, 28.11% |
6.574, 43.45% |
|||
High volume Mean |
163 |
113.7 |
532.3 |
372.1 |
1259.5 |
883.8 |
Variance |
207.333 |
86.233 |
1108.456 |
485.433 |
623.167 |
1494.622 |
t-value(95%), difference (%) |
9.099, 30.25% |
12.689, 30.1% |
25.817, 29.83% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table indicates that the right-turn bay reduces the total number of conflict events for most levels of traffic volume and threshold for the TTC value.
Crossing |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NRB | WRB | NRB | WRB | NRB | WRB | |
Medium volume Mean |
0.5 |
0.6 |
0.7 |
0.8 |
1.3 |
0.8 |
Variance |
0.944 |
0.933 |
1.122 |
1.511 |
1.567 |
1.511 |
t-value(95%), difference (%) |
-0.231 |
-0.195 |
0.901 |
|||
High volume Mean |
6.5 |
7.3 |
9 |
10.8 |
14.3 |
15.3 |
Variance |
6.278 |
3.122 |
7.778 |
7.067 |
11.789 |
11.789 |
t-value(95%), difference (%) |
-0.825 |
-1.477 |
-0.651 |
As expected this table indicates that a right-turn bay would not reduce the number of crossing-conflict events.
Rear End | TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NRB | WRB | NRB | WRB | NRB | WRB | |
Low volume Mean | 3.3 |
2.1 |
11.6 |
7.7 |
25.8 |
12.6 |
Variance | 3.344 |
1.656 |
14.044 |
9.789 |
23.733 |
13.156 |
t-value(95%), difference (%) | 1.697 |
2.526, 33.62% |
6.873, 51.16% |
|||
Medium volume Mean | 5 |
3.3 |
29.6 |
18.7 |
92.1 |
44.4 |
Variance | 5.556 |
4.678 |
76.044 |
22.011 |
252.544 |
99.600 |
t-value(95%), difference (%) | 1.681 |
3.481, 36.82% |
8.038, 51.79% |
|||
High volume Mean | 97.6 |
60 |
409.9 |
262.9 |
1024.9 |
675.6 |
Variance | 108.044 |
81.111 |
922.322 |
360.322 |
468.767 |
1380.711 |
t-value(95%), difference (%) | 8.645, 38.52% |
12.98, 35.86% |
25.685, 34.08% |
Note: Shaded cells indicate statistically significant differences between the two alternatives.
This table indicates that adding a right-turn bay will statistically reduce the number of rear-end conflicts for all traffic volumes, as expected from field experience.
Lane Change |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
NRB | WRB | NRB | WRB | NRB | WRB | |
Low volume Mean |
2.5 |
1.4 |
4.7 |
7 |
9.6 |
11.4 |
Variance |
2.056 |
1.600 |
2.233 |
5.556 |
5.156 |
13.156 |
t-value(95%), difference (%) |
1.819 |
-2.606, -48.94% | -1.330 |
|||
Medium volume Mean |
6.5 |
6.2 |
13.1 |
11.7 |
25.6 |
22.1 |
Variance |
6.722 |
8.622 |
15.656 |
28.456 |
30.044 |
66.544 |
t-value(95%), difference (%) |
0.242 |
0.667 |
1.126 |
|||
High volume Mean |
58.9 |
46.4 |
113.4 |
98.4 |
220.3 |
192.9 |
Variance |
82.322 |
4.711 |
170.044 |
20.267 |
235.122 |
169.656 |
t-value(95%), difference (%) |
4.237, 21.22% | 3.438, 13.23% | 4.307, 12.44% |
Note: Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
This table indicates that adding a right-turn bay will statistically reduce the number of lane change conflicts for some traffic volumes. Notably, at high volumes, the number of high-severity (indicated by considering only TTC values less than 0.5s and 1.0s) lane-change events is reduced, as expected from field experience.
TNRB | TWRB | CNRB | CWRB | RENRB | REWRB | LCNRB | LCWRB | |
---|---|---|---|---|---|---|---|---|
TTC (low) |
1.03 |
0.97 |
N/A |
N/A |
1.05 |
0.96 |
0.94 |
0.97 |
t -value, diff(%) |
1.887 |
N/A |
2.213, 8.57% | -0.504 |
||||
TTC (med) |
1.11 |
1.02 |
0.82 |
0.49 |
1.17 |
1.09 |
0.92 |
0.9 |
t -value, diff(%) |
4.625, 8.11% | 1.378 |
4.115, 6.84% | 0.445 |
||||
TTC (high) |
1.05 |
1.05 |
0.71 |
0.65 |
1.08 |
1.09 |
0.9 |
0.92 |
t -value, diff(%) |
0.000 |
1.012 |
-1.842 |
-1.339 |
||||
PET(low) |
2.16 |
1.88 |
N/A |
N/A |
2.28 |
2.09 |
1.78 |
1.62 |
t -value, diff(%) |
2.569, 12.96% | N/A |
1.362 |
0.884 |
||||
PET(med) |
2.49 |
2.22 |
1.68 |
0.26 |
2.77 |
2.53 |
1.52 |
1.69 |
t -value, diff(%) |
4.315, 10.84% | 2.687, 84.52% | 3.545, 8.66% | -1.462 |
||||
PET(high) |
2.12 |
2.14 |
0.85 |
0.75 |
2.3 |
2.4 |
1.35 |
1.32 |
t -value, diff(%) |
-1.165 |
1.042 |
-5.378, -4.35% | 0.918 |
||||
MaxS(low) |
30.74 |
32.3 |
N/A |
N/A |
31.1 |
31.34 |
29.99 |
33.61 |
t -value, diff(%) |
-1.925 |
N/A |
-0.212 |
-2.676, -12.07% | ||||
MaxS(med) |
27.07 |
26.33 |
25.48 |
27.73 |
25.83 |
24.3 |
31.59 |
30.37 |
t -value, diff(%) |
1.616 |
-0.553 |
2.946, 5.92% | 1.380 |
||||
MaxS(high) |
24.67 |
25.15 |
21.32 |
23.43 |
24.75 |
25.28 |
24.55 |
24.84 |
t -value, diff(%) |
-4.347, -1.95% | -3.162, -9.90% | -4.218, -2.14% | -1.181 |
||||
DeltaS(low) |
26.28 |
29.12 |
N/A |
N/A |
27.84 |
29.01 |
22.13 |
29.45 |
t -value, diff(%) |
-3.004, -10.81% | N/A |
-0.907 |
-4.854, -33.08% | ||||
DeltaS(med) |
17.9 |
17.84 |
23.98 |
26.08 |
17.24 |
16.46 |
19.96 |
20.31 |
t -value, diff(%) |
0.101 |
-0.515 |
1.054 |
-0.348 |
||||
DeltaS(high) |
18.29 |
18.58 |
19.35 |
21.89 |
18.95 |
19.44 |
15.14 |
15.29 |
t -value, diff(%) |
-2.165, -1.59% | -3.382, -13.13% | -3.253, -2.59% | -0.515 |
||||
DR(low) |
-6.96 |
-6.48 |
N/A |
N/A |
-7.29 |
-6.59 |
-5.71 |
-6.22 |
t -value, diff(%) |
-1.662 |
N/A |
-1.925 |
1.010 |
||||
DR(med) |
-5.66 |
-4.69 |
-7.27 |
-3.97 |
-5.9 |
-4.99 |
-4.7 |
-4.12 |
t -value, diff(%) |
-4.732, 17.14% | -1.052 |
-3.670, 15.42% | -1.489 |
||||
DR(high) |
-5.17 |
-5.16 |
-0.83 |
-0.93 |
-5.75 |
-5.96 |
-2.78 |
-2.7 |
t -value, diff(%) |
-0.176 |
0.320 |
3.440, -3.65% | -0.667 |
||||
MaxD(low) |
-14.54 |
-15.33 |
N/A |
N/A |
-15.54 |
-16.13 |
-11.66 |
-14.39 |
t -value, diff(%) |
1.980, 5.43% | N/A |
1.164 |
3.866, -23.41% | ||||
MaxD(med) |
-13.19 |
-12.28 |
-9.69 |
-4.48 |
-13.95 |
-13.41 |
-10.62 |
-10.29 |
t -value, diff(%) |
-3.977, 6.90% | -1.558 |
-2.852, 3.87% | -0.558 |
||||
MaxD(high) |
-13.03 |
-12.72 |
-3.82 |
-3.14 |
-14.23 |
-14.33 |
-8.09 |
-7.86 |
t -value, diff(%) |
-4.475, 2.38% | -1.158 |
1.924 |
-1.150 |
||||
MaxDeltaV(low) |
14.83 |
16.75 |
N/A |
N/A |
15.8 |
16.7 |
12.21 |
16.92 |
t -value, diff(%) |
-3.278, -12.95% | N/A |
-1.128 |
-5.199, -38.57% | ||||
MaxDeltaV(med) |
10.11 |
10.07 |
13.59 |
14.13 |
9.72 |
9.28 |
11.32 |
11.52 |
t -value, diff(%) |
0.115 |
-0.227 |
1.024 |
-0.342 |
||||
MaxDeltaV(high) |
10.19 |
10.37 |
10.89 |
12.52 |
10.54 |
10.82 |
8.54 |
8.62 |
t -value, diff(%) |
-2.357, -1.77% |
-3.428, -14.97% |
-3.263, -2.66% |
-0.478 |
Note: Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
Similar to the average value results for the left-turn bay, the existence of the right-turn bay tends to make the severity of conflicts worse for measures other than the primary measure TTC.
Correlations with Predicted Crash Frequency
The predicted crash rates for all scenarios in this test are listed in table 28 with the corresponding surrogate measures of safety. Rank orders for each category of data are also listed in the table. The Spearman rank correlation coefficients are calculated for each test.
AADT |
Low |
Medium |
High |
Rs | ||||
---|---|---|---|---|---|---|---|---|
NRB | WRB | NRB | WRB | NRB | WRB | |||
Crash Frequency |
M |
5.5 |
5.2 |
8 |
7.5 |
8.7 |
8.3 |
1 |
R |
2 |
1 |
4 |
3 |
6 |
5 |
||
Total Conflict |
M |
35.9 |
24.2 |
119 |
67.3 |
1,259.5 |
883.8 |
0.97 |
R |
2 |
1 |
4 |
3 |
5 |
5 |
||
Crossing Conflict |
M |
0.5 |
0.2 |
1.3 |
0.8 |
14.3 |
15.3 |
0.91 |
R |
1 |
1 |
3 |
3 |
5 |
5 |
||
Rear-End Conflict |
M |
25.8 |
12.6 |
92.1 |
44.4 |
1,024.9 |
675.6 |
0.97 |
R |
2 |
1 |
4 |
3 |
5 |
5 |
||
LC Conflict |
M |
9.6 |
11.4 |
25.6 |
22.1 |
220.3 |
192.9 |
0.80 |
R |
1 |
1 |
3 |
3 |
5 |
5 |
Note: Rows labeled "M" provide mean values and rows labeled "R" provide the ranking of each alternative. The Rs column provides Spearman rank correlation coefficients indicating agreement with theoretical crash estimates.
As seen in the table above, the correlation between the number of conflicts and the crash prediction model is very high.
Findings and Conclusions
Based on the observation on the safety surrogate data obtained from the test, the following conclusions can be drawn:
In general, an intersection with a right-turn bay experiences less total, rear-end, and lane-change conflicts than an intersection without a right-turn bay. Rear-end conflicts constitute the major part of the total conflicts (ranging from 50 to 80 percent) and have larger TTC and PET values (≥ 1.0) than other types of conflicts. There is no significant difference for the number of crossing conflicts between these two intersection designs. These results match what would generally be expected from field experience.
The Spearman-rank correlation coefficients resulted from all tests show a strong positive relationship between the rank orders of the surrogate measures of safety and the rank orders of the predicted crash rates, with only slight differences between the correlation coefficients for the various individual conflict types.
Leading left turn and lagging left turn are two different control logics for protected left turns. A leading left turn allows the green phase of left-turn movements ahead of the green phase of through movements, while a lagging left turn places the green phase of the left turn after the green phase of through movements. At the beginning of the study, there is no determinate evidence that either type of left-turn operation has an appreciable effect on the safety of the intersection. Several combinations of the left-turn to through-volume ratio have been evaluated for each design alternative (leading versus lagging).
Until now, no considerations of control logic for protected left turn have been included in crash prediction models for conventional intersections, so no rank order comparison can be performed.
Intersection Description
The intersection used to test leading left turn versus lagging left turn is a four-legged intersection with one through lane having left- and right-turn bays in the main travel directions and one though lane on the side street, as shown in figure 52 and figure 53. All left- turn bays are 76.25 m (250 ft) long.
Figure 52. Screen Capture. Intersection with Leading Left Turn.
Figure 53. Screen Capture. Intersection with Lagging Left Turn.
Table 29 shows the traffic volumes applied for each approach of the intersection. Fixed time traffic control is applied in this test. Figure 54 through figure 59 provide the timing plans used for each testing scenario (low to high volumes).
Approach |
Southbound | Northbound | Eastbound | Westbound | ||||||||
L | TH | R | L | TH | R | L | TH | R | L | TH | R | |
Phase ID | 3 |
3 |
4 |
4 |
1 |
2 |
1 |
2 |
||||
Low Volumes | 25 |
75 |
25 |
50 |
100 |
50 |
150 |
400 |
50 |
150 |
400 |
50 |
Medium Volumes | 25 |
75 |
25 |
50 |
100 |
50 |
125 |
650 |
75 |
125 |
650 |
75 |
High Volumes | 100 |
250 |
50 |
100 |
250 |
50 |
150 |
700 |
150 |
150 |
700 |
150 |
Note: L, TH, and R correspond to vehicles proceeding left, through, or right at the intersection.
Figure 54. Illustration. Timing Plan for Intersection with Lag Left Turn in Low Volumes (Cycle: 80; Split: 22, 17, 22, and 19).
Figure 55. Illustration. Timing Plan for Intersection with Lead Left Turn in Low Volumes (Cycle: 80; Split: 17, 22, 22, and 19).
Figure 56. Illustration. Timing Plan for Intersection with Lag Left Turn in Medium Volumes (Cycle: 80; Split: 33, 17, 15, and 15).
57. Illustration. Timing Plan for Intersection with Lead Left Turn in Medium Volumes (Cycle: 80; Split: 17, 33, 15, and 15).
Figure 58. Illustration. Timing Plan for Intersection with Lag Left Turn in High Volumes (Cycle: 75; Split: 20, 11, 23, and 21).
Figure 59. Illustration. Timing Plan for Intersection with Lead Left Turn in High Volumes (Cycle: 75; Split: 11, 20, 23, and 21).
Data Analysis and Comparison Results
Ten replications were performed for each simulation scenario, and the resulting output trajectory data was analyzed by SSAM. The F-test and t-test were applied to compare the average number of conflict events and surrogate measures of safety from one scenario to the other.
Table 30 through table 33 provide the values of all surrogate measures of safety and corresponding t-test results for different types of aggregations with the low-speed events and crash data excluded (TTC ≤ 0 and MaxS ≥ 16.1 km/h (10 mi/h)).
Total |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
Lead | Lag | Lead | Lag | Lead | Lag | |
Low volume Mean |
N/A |
N/A |
N/A |
N/A |
9.90 |
8.80 |
Variance |
N/A |
N/A |
N/A |
N/A |
5.43 |
14.84 |
t-value(95%), difference (%) |
N/A |
N/A |
0.772 |
|||
Medium volume Mean |
N/A |
N/A |
N/A |
N/A |
16.30 |
13.40 |
Variance |
N/A |
N/A |
N/A |
N/A |
12.23 |
23.60 |
t-value(95%), difference (%) |
N/A |
N/A |
1.532 |
|||
High volume Mean |
N/A |
N/A |
8.10 |
8.10 |
35.60 |
32.30 |
Variance |
N/A |
N/A |
5.88 |
11.88 |
78.27 |
33.34 |
t-value(95%), difference (%) |
N/A |
0 |
0.988 |
Rear End | TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
Lead | Lag | Lead | Lag | Lead | Lag | |
Low volume Mean | N/A |
N/A |
N/A |
N/A |
5.70 |
5.70 |
Variance | N/A |
N/A |
N/A |
N/A |
3.57 |
8.90 |
t-value(95%), difference (%) | N/A |
N/A |
0 |
|||
Medium volume Mean | N/A |
N/A |
N/A |
N/A |
10.00 |
8.60 |
Variance | N/A |
N/A |
N/A |
N/A |
7.78 |
13.60 |
t-value(95%), difference (%) | N/A |
N/A |
0.958 |
|||
High volume Mean | N/A |
N/A |
5.80 |
5.80 |
24.30 |
22.10 |
Variance | N/A |
N/A |
3.96 |
6.18 |
42.01 |
21.21 |
t-value(95%), difference (%) | N/A |
0 |
0.875 |
Lane Change |
TTC ≤ 0.5 | TTC ≤ 1.0 | TTC ≤ 1.5 | |||
---|---|---|---|---|---|---|
Lead | Lag | Lead | Lag | Lead | Lag | |
Low volume Mean |
N/A |
N/A |
N/A |
N/A |
4.10 |
3.10 |
Variance |
N/A |
N/A |
N/A |
N/A |
5.21 |
2.54 |
t-value(95%), difference (%) |
N/A |
N/A |
1.136 |
|||
Medium volume Mean |
N/A |
N/A |
N/A |
N/A |
6.20 |
4.50 |
Variance |
N/A |
N/A |
N/A |
N/A |
6.40 |
3.17 |
t-value(95%), difference (%) |
N/A |
N/A |
1.738 |
|||
High volume Mean |
N/A |
N/A |
2.20 |
2.30 |
11.20 |
9.90 |
Variance |
N/A |
N/A |
2.84 |
3.12 |
15.29 |
6.99 |
t-value(95%), difference (%) |
N/A |
-0.129 |
0.871 |
TLead | TLag | CLead | CLag | RELead | RELag | LCLead | LCLag | |
---|---|---|---|---|---|---|---|---|
TTC (low) |
1.28 |
1.28 |
N/A |
N/A |
1.32 |
1.3 |
1.23 |
1.25 |
t -value, diff(%) |
0.000 |
N/A |
0.571 |
-0.305 |
||||
TTC (med) |
1.27 |
1.29 |
N/A |
N/A |
1.31 |
1.28 |
1.2 |
1.28 |
t -value, diff(%) |
-0.868 |
N/A |
1.281 |
-1.533 |
||||
TTC (high) |
1.2 |
1.2 |
N/A |
N/A |
1.2 |
1.19 |
1.2 |
1.24 |
t -value, diff(%) |
0.000 |
N/A |
0.46 |
-1.240 |
||||
PET(low) |
2.1 |
2.02 |
N/A |
N/A |
2.27 |
2.13 |
1.89 |
1.83 |
t -value, diff(%) |
0.695 |
N/A |
1.102 |
0.272 |
||||
PET(med) |
2.3 |
2.35 |
N/A |
N/A |
2.42 |
2.52 |
2.1 |
1.99 |
t -value, diff(%) |
-0.497 |
N/A |
-0.860 |
0.607 |
||||
PET(high) |
2.47 |
2.47 |
N/A |
N/A |
2.48 |
2.58 |
2.48 |
2.23 |
t -value, diff(%) |
0.000 |
N/A |
-1.467 |
2.154, 10.08% | ||||
MaxS(low) |
6.69 |
6.78 |
N/A |
N/A |
6.37 |
6.72 |
7.16 |
6.9 |
t -value, diff(%) |
-0.242 |
N/A |
-0.763 |
0.405 |
||||
MaxS(med) |
7.52 |
7.19 |
N/A |
N/A |
6.94 |
7.53 |
8.47 |
6.6 |
t -value, diff(%) |
0.964 |
N/A |
-1.331 |
3.834, 22.1% | ||||
MaxS(high) |
7.76 |
7.94 |
N/A |
N/A |
7.78 |
7.8 |
7.78 |
8.24 |
t -value, diff(%) |
-0.734 |
N/A |
-0.064 |
-1.036 |
||||
DeltaS(low) |
5.35 |
5.25 |
N/A |
N/A |
5.28 |
5.25 |
5.45 |
5.26 |
t -value, diff(%) |
0.576 |
N/A |
0.195 |
0.481 |
||||
DeltaS(med) |
5.37 |
5.35 |
N/A |
N/A |
5.24 |
5.21 |
5.56 |
5.59 |
t -value, diff(%) |
0.135 |
N/A |
0.184 |
-0.1015 |
||||
DeltaS(high) |
4.47 |
4.61 |
N/A |
N/A |
4.22 |
4.3 |
4.22 |
5.23 |
t -value, diff(%) |
-0.986 |
N/A |
-0.462 |
-4.079, -23.93% | ||||
DR(low) |
-2.48 |
-2.24 |
N/A |
N/A |
-2.62 |
-2.23 |
-2.28 |
-2.25 |
t -value, diff(%) |
-1.781 |
N/A |
-2.357, 14.89% | -0.129 |
||||
DR(med) |
-2.28 |
-2.44 |
N/A |
N/A |
-2.31 |
-2.34 |
-2.22 |
-2.51 |
t -value, diff(%) |
1.095 |
N/A |
0.230 |
0.871 |
||||
DR(high) |
-1.9 |
-1.97 |
N/A |
N/A |
-1.72 |
-1.85 |
-1.72 |
-2.21 |
t -value, diff(%) |
0.624 |
N/A |
1.057 |
2.371, -28.49% | ||||
MaxD(low) |
-3.42 |
-3.05 |
N/A |
N/A |
-3.2 |
-2.9 |
-3.65 |
-3.32 |
t -value, diff(%) |
-1.573 |
N/A |
-1.177 |
-0.743 |
||||
MaxD(med) |
-3.54 |
-3.79 |
N/A |
N/A |
-3.26 |
-3.83 |
-3.94 |
-3.61 |
t -value, diff(%) |
1.117 |
N/A |
2.379, -17.48% | -0.737 |
||||
MaxD(high) |
-4.13 |
-4.02 |
N/A |
N/A |
-4.02 |
-4.02 |
-4.02 |
-4 |
t -value, diff(%) |
-0.805 |
N/A |
0.000 |
-0.077 |
||||
MaxDeltaV(low) |
2.78 |
2.68 |
N/A |
N/A |
2.78 |
2.62 |
2.79 |
2.77 |
t -value, diff(%) |
1.027 |
N/A |
1.801 |
0.094 |
||||
MaxDeltaV(med) |
2.74 |
2.7 |
N/A |
N/A |
2.66 |
2.6 |
2.87 |
2.86 |
t -value, diff(%) |
0.452 |
N/A |
0.675 |
0.051 |
||||
MaxDeltaV(high) |
2.28 |
2.36 |
N/A |
N/A |
2.15 |
2.2 |
2.15 |
2.68 |
t -value, diff(%) |
-1.089 |
N/A |
-0.559 |
-4.107, -24.65% |
Note: Shaded cells indicate statistically significant differences between the two alternatives. The tan and blue colors indicate extreme values to the right and left columns respectively.
These tables indicate no significant differences between either the number or severity of conflict events for leading and lagging protected left turns, as expected from field experience.
Correlations with Predicted Crash Frequency
Since no consideration for leading or lagging left turns has been incorporated into a crash prediction model to date, any comparisons do not have meaning. Results for crossing conflicts have been excluded from the table below because there were not enough events to analyze (left turns were protected only).
AADT |
Low | Medium | High | Rs | ||||
---|---|---|---|---|---|---|---|---|
Lead | Lag | Lead | Lag | Lead | Lag | |||
Crash Frequency |
M | 3.9 |
3.9 |
4.8 |
4.8 |
6.3 |
6.3 |
1 |
R | 1 |
1 |
3 |
3 |
5 |
5 |
||
Total Conflict |
M | 9.90 |
8.80 |
16.3 |
13.4 |
35.60 |
32.30 |
1 |
R | 1 |
1 |
3 |
3 |
5 |
5 |
||
Crossing Conflict |
M | > |
N/A |
|||||
R | > |
> |
||||||
Rear-End Conflict |
M | 5.7 |
5.7 |
10 |
8.6 |
24.30 |
22.10 |
1 |
R | 1 |
1 |
3 |
3 |
5 |
5 |
||
LC Conflict |
M | 4.1 |
3.1 |
6.20 |
4.50 |
11.20 |
9.90 |
1 |
R | 1 |
1 |
3 |
3 |
5 |
5 |
Note: Rows labeled "M" provide mean values and rows labeled "R" provide the ranking of each alternative. The Rs column provides Spearman rank correlation coefficients indicating agreement with theoretical crash estimates.
Findings and Conclusions
Based on the observation on the safety surrogate data obtained from the test, the following conclusions can be drawn:
In general, there is no significant difference for any of the surrogate measures of safety between leading protected left turns and lagging protected left turns. This result matches the intuitive expectation. There may, however, be some difference between the two if a permitted-protected operation was considered instead of a protected-only operation.
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FHWA-HRT-08-051 |