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Safety Performance of Priority
Three Leg Intersections –
Seagulls and Left Turn Slip
Lanes
Submitted to
AITPM Conference
J...
Presentation Overview
• Reason for research
• NZ and overseas research
• Dataset size
• Predictor variables
• Crash modell...
Reason for Research
• There has been a history of poor crash performance at some seagull intersections – in
some jurisdict...
Current Knowledge – NZ Studies
• Crash prediction models are available for both urban (low speed) and rural
(high speed – ...
Local Literature – Arndt and Turner
• Arndt 2005 - Key Variables (143 T-intersections + 63 X-roads)
• Visibility between m...
Literature - Seagulls
• Radalj, et al., (2006) analysed 76 seagull intersections in Perth. The
study identified that seagu...
Literature - Seagulls
• Harper, et al., (2011) researched the safety performance of three
design variations of a seagull i...
Literature - LTSLs
• Research undertaken by Ale et al (2013) identified that the provision
of left turn lanes reduces the ...
Datasets
Intersection Type Urban Rural
Standard Tees 92 92
Seagulls 17 12
Tees with LTSL 10 37
TOTAL 119 141
T-Junction Crash Types
Rural
Urban
JA Crashes
34%
LB Crashes
3%
10%
53%
Rural T-Junction Crashes
JA Crashes LB Crashes GD ...
Rural Intersections
JA Crashes
24%
LB Crashes
24%
15%
37%
Rural T-Junction LTSL (37)
JA Crashes LB Crashes
GD Crashes Othe...
Urban Intersections
JA Crashes
31%
LB Crashes
15%8%
46%
Urban T-Junction LTSL (10)
JA Crashes LB Crashes
GD Crashes Other ...
Crash Prediction Model
JA crashes at standard urban T-Junctions
Crashes = 6.760 × 10−12
× 𝑄1
0.19
× 𝑄5
0.23
× 𝑀𝑅𝑆𝐿3.8
× 𝑈𝐽...
Key Variables
Flows
Intersection Size
• No. through lanes
• Side Road approach lanes
• Side Road median
Distractions/Press...
JA Crash Models –Urban Tee
1. More traffic and higher speeds less
safe
2. Bigger Intersections less safe
3. Features (park...
LB Crash Models – Urban Tee
1. More traffic and higher speeds less
safe
2. Bigger Intersections and parking on
RT approach...
Urban Model Factors EXPOSURE | SPEED | VISIBILITY |
DISTRACTION | SIZE | COMPLEXITY
Factors - Model Standard
JA
Standard
L...
JA Crash Models – Standard Rural Tee
1. More traffic and higher speeds less
safe
2. Bigger Intersections less safe
3. Feat...
JA Crash Models – Rural Seagull or LTSL
1. More traffic and higher speeds less
safe
2. Seagulls less safe when 4-lanes,
wi...
LB Crash Models – Rural Tee with LTSL
1. More traffic and higher speeds less
safe
2. Poor left turn lane off-set reduces
v...
Rural Model Factors EXPOSURE | SPEED | VISIBILITY |
DISTRACTION | SIZE | COMPLEXITY
Factors - Model Standard JA LTSL JA LT...
Areas of Refinement
• Explore further the impact of wide medians on seagull
performance – may be related to RT bay entry a...
Questions?
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The Safety Performance of Priority Three-leg Iintersections: seagulls and left turn slip lanes

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Dr Shane Turner, Mike Smith and Prof Graham Wood

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The Safety Performance of Priority Three-leg Iintersections: seagulls and left turn slip lanes

  1. 1. Safety Performance of Priority Three Leg Intersections – Seagulls and Left Turn Slip Lanes Submitted to AITPM Conference July 2016
  2. 2. Presentation Overview • Reason for research • NZ and overseas research • Dataset size • Predictor variables • Crash modelling results
  3. 3. Reason for Research • There has been a history of poor crash performance at some seagull intersections – in some jurisdictions this concern means that there is a reluctance to use this intersection option despite the efficiency benefits. • There are concerns that some left turn slip lane treatment designs may increase crash risk especially where the left turn in and right turn out movements are high – the international research is not conclusive that left turn slip lanes improve road safety. • There is limited research available on the safety performance of high volume priority T- intersections – the crash prediction models manly cover lower traffic volumes sites where the entering traffic has plenty of gaps in the main traffic flow. It would be useful to understand the contribution of design factors, main road speed and traffic flows on crash occurrence. • Key outcome is to expand the current NZ crash prediction models for this intersection type to include seagull layouts and LTSLs
  4. 4. Current Knowledge – NZ Studies • Crash prediction models are available for both urban (low speed) and rural (high speed – 80km/h plus) priority intersections • These models are presented in the NZTA Economic Evaluation Manual as a Compendium to the Crash Analysis Section (this is the Crash Estimation Compendium) • Basic product of flow models (total crashes as function of daily two way traffic on each road) are available for both urban and rural Tees. • AT = b0 × Qmajor b1 × Qminor/side b2 • For rural tees conflicting flows models are also available for each major crash type. These models also include non-flow predictors, including operating speed, visibility and presence of right turn bay (RTB). • AT = 1.08 × 10-6 × q4 0.36 × q5 1.08× ΦRTB • ΦRTB = 0.22 (if right-turn bay present)
  5. 5. Local Literature – Arndt and Turner • Arndt 2005 - Key Variables (143 T-intersections + 63 X-roads) • Visibility between minor and major road vehicles • Visibility between right turners and through vehicles on main road • Minor and Major road approach speeds (85%ile) • Number of lanes on side-road • Right turn provision – widening to RTB • Observation angle SKEW (degree at which drivers have to look backwards) • Turner 2007 – Key Variables (100 Rural T-intersections) • Visibility from side-road • Approach speed on main road • Presence of RTB
  6. 6. Literature - Seagulls • Radalj, et al., (2006) analysed 76 seagull intersections in Perth. The study identified that seagull intersections • installed as per the recommended guidelines, do not result in any significant (positive or negative) change in the type or number of crashes. • installed with a seagull entry angle that did not conform to the recommended guidance (55 to 70 degrees), had more crashes and higher crash severity, especially the latter. • Elvik, et al., (2009) based on a review several European and USA studies has concluded that chanelised passing lanes at T-junctions (seagull equivalent) increases the crash risk by 26%. • Summersgill, et al., (1996) investigated the frequency and character of crashes at priority intersections in the UK. They found an increase in ‘JA’ crashes of 50% at chanelised intersections.
  7. 7. Literature - Seagulls • Harper, et al., (2011) researched the safety performance of three design variations of a seagull intersection design for the A1 Highway / Island Point Road intersection in New South Wales, Australia. • After the seagull intersection was constructed a number of ‘right near’ (JA) type crashes began to occur. • The intersection was modified to include a short left turn splay (LTSL) that included a small raised concrete splitter island and priority control. This did not reduce JA crashes but increased LB crashes (from 2 to 6 injury crashes per year). • A final modification increased separation between the left-turn deceleration lane and the straight through lane of the major road. After which the crashes reduced appreciably (I per year).
  8. 8. Literature - LTSLs • Research undertaken by Ale et al (2013) identified that the provision of left turn lanes reduces the incidence of rear-end crashes, the crash severity and the associated economic costs. • Elvik et al (2009) identified from several studies that the provision of left turn lanes at T-junctions acts to increase the number of injuries by 12%. • They reasoned that left turn lanes may create blind spots where a vehicle turning left can obscure approaching through traffic. • They also added that large scale intersection channelisation can complicate the road layout, which may increase driver error and in particular cause more severe ‘JA’ crashes. • Highway Safety Manual (ASSHTO) shows a crash reduction
  9. 9. Datasets Intersection Type Urban Rural Standard Tees 92 92 Seagulls 17 12 Tees with LTSL 10 37 TOTAL 119 141
  10. 10. T-Junction Crash Types Rural Urban JA Crashes 34% LB Crashes 3% 10% 53% Rural T-Junction Crashes JA Crashes LB Crashes GD Crashes Other Crashes JA Crashes 21% LB Crashes 17% 4% 58% Urban T-Junction Crashes JA Crashes LB Crashes GD Crashes Other Crashes
  11. 11. Rural Intersections JA Crashes 24% LB Crashes 24% 15% 37% Rural T-Junction LTSL (37) JA Crashes LB Crashes GD Crashes Other Crashes JA Crashes 50% LB Crashes 37% 0% 13% Rural Seagulls JA Crashes LB Crashes GD Crashes Other Crashes JA Crashes 34% LB Crashes 3% 10% 53% Rural T-Junction Crashes (92) JA Crashes LB Crashes GD Crashes Other Crashes
  12. 12. Urban Intersections JA Crashes 31% LB Crashes 15%8% 46% Urban T-Junction LTSL (10) JA Crashes LB Crashes GD Crashes Other Crashes JA Crashes 33% LB Crashes 22% 0% 45% Urban Seagulls (17) JA Crashes LB Crashes GD Crashes Other Crashes JA Crashes 21% LB Crashes 17% 4% 58% Urban T-Junction Crashes (92) JA Crashes LB Crashes GD Crashes Other Crashes
  13. 13. Crash Prediction Model JA crashes at standard urban T-Junctions Crashes = 6.760 × 10−12 × 𝑄1 0.19 × 𝑄5 0.23 × 𝑀𝑅𝑆𝐿3.8 × 𝑈𝐽𝐴𝐷𝐼2.9 Where, 𝑄1 = Right turn from side road 𝑄5 = Right to left movement on main road MRSL= Main Road Speed Limit UJADI = Design index (seven features) Model does not fit without design index
  14. 14. Key Variables Flows Intersection Size • No. through lanes • Side Road approach lanes • Side Road median Distractions/Pressure • Right turn bay stacking length • Type of feature • Distance to feature Speed Limits (50, 60, 70, 80, 100) Geometry • Main Rd Gradient • Seagull entry splitter island length • LTSL Distance to diverge • LTSL Offset
  15. 15. JA Crash Models –Urban Tee 1. More traffic and higher speeds less safe 2. Bigger Intersections less safe 3. Features (parking and side-roads) to the left cause distraction 4. Seagulls - more crashes where wider main road median
  16. 16. LB Crash Models – Urban Tee 1. More traffic and higher speeds less safe 2. Bigger Intersections and parking on RT approach are less safe 3. Longer splitter island length is safer 4. Late left turns are safer at Seagulls than early left turns (removes confusion?)
  17. 17. Urban Model Factors EXPOSURE | SPEED | VISIBILITY | DISTRACTION | SIZE | COMPLEXITY Factors - Model Standard JA Standard LB Seagull JA Seagull LB Conflicting Flows X X X X MRSL X X X X No Through lanes X X X X No Approach Lanes X X X X Distance to Feature X X X X SR LT Control Type X MR Median Width X X SR Median Width X SR Median (Y/N) X X Seagull entry splitter length X MR gradient X X LTSL Distance to diverge X
  18. 18. JA Crash Models – Standard Rural Tee 1. More traffic and higher speeds less safe 2. Bigger Intersections less safe 3. Features (side-roads) to the left cause distraction 4. More crashes where longer right turn bay (more right turners) 5. Side road gradient is less safe
  19. 19. JA Crash Models – Rural Seagull or LTSL 1. More traffic and higher speeds less safe 2. Seagulls less safe when 4-lanes, wider median and RT stacking 3. Poor left turn lane off-set reduces visibility and increases crashes 4. Late left turn drop safer for LTSL 5. Features (side-roads) to the left cause distraction when LTSL
  20. 20. LB Crash Models – Rural Tee with LTSL 1. More traffic and higher speeds less safe 2. Poor left turn lane off-set reduces visibility and increases crashes 3. Width of side-road (no. lanes and presence of median) increase crashes. 4. LTSL design impacts on crashes 5. More crashes where longer right turn bay (more right turners)
  21. 21. Rural Model Factors EXPOSURE | SPEED | VISIBILITY | DISTRACTION | SIZE | COMPLEXITY Factors - Model Standard JA LTSL JA LTSL LB Seagull JA Seagull LB Conflicting Flows X X X X X MRSL X X X X X No. Through lanes X Limited data Limited data X X LTSL Offset (Visibility past LT) N/A X X X X Distance to Feature X X MR Median Width Limited data Limited data Limited data X X RTB Stacking Length X X X Chevron Board (Y/N) X X LTSL Type N/A X X Number of other factors 2 3 3 3 0
  22. 22. Areas of Refinement • Explore further the impact of wide medians on seagull performance – may be related to RT bay entry angle • Examine wet weather and night crash percentages. Also look at time of crashes (in peaks?) • Rural Tees – why is feature upstream an issue for Rural JA’s. Is it distraction/less attention to right turn traffic or is it the complexity of a staggered intersection? • All the design index variables are currently weighted the same. Need to consider more weight on stronger design variables. • Consider bias by selection (eg chevron boards & side road medians are often installed at high crash risk sites).
  23. 23. Questions?

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