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High risk intersections - making sense of safe systems

in practice

Tim Hughes – National safety engineer

Safer journeys action plan

Improve high-risk intersections We will use the High Risk Intersection Guide to

identify and target the 100 highest-risk intersections to address by 2020.

In 2013 a high-risk intersection programme will be developed and launched.

Over 2014/15 the following will be delivered:• By September 2014 solutions will be developed

for at least 30 high-risk intersections. • By June 2015 at least 20 high-risk intersections

will be improved as per agreed solutions.

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High Risk Intersections - Top 100 list

40 are at traffic signals37 of these are urban signals / 47 urban total

Only 3 are rural signals.

However: we also have a top 700 list based on collective risk and there will be a similar number on personal risk.

Some of these may be better value for money to fix than the top 100. HRIG helps you to identify and prioritise.

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The Safer Journeys Safe System Vision

A safe road system

increasingly free of

death and serious injury

Safe Speed Thresholds

Roads with possible conflicts between vehicles and unprotected users 30-40 km/h

Intersections with possible side-on conflicts between vehicles 50 km/h

Roads with possible frontal conflicts between vehicles 70 km/h

Roads with no likelihood of frontal or side-on conflicts between road users 100 km/h

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The challenge of a safe system

• The focus on deaths and serious injuries• Identifying high risk locations• Crash types that result in deaths and serious

injuries (DSIs)• How do countermeasures affect crash severity• Crash prediction models and crash severity /

casualties• Prioritising on saving DSIs• Safety Audit

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Structure of HRIG

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Section 2 Strategic Context

Section 3 Crash Priorities

Section 4 Identifying HRI

Section 5 Understanding the issues

Section 6 Intersection Countermeasures

Section 7 Programme, Monitoring and Evaluation

Appendices Detailed tables, countermeasures and references

Severity by Side impact speed

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Severity by Speed limit

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Severity by Speed limit

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0.00

0.05

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0.25

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0.35

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Urban Rural

DIS

s p

er in

jury

cra

sh

Speed Limit

Crash severity by intersection type and speed limit

X-priority

X-signals

T-priority

T-signals

Roundabout

Main crash types - urban

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CRASH TYPE

Head On Off road straight

Off Road bend

Rear end turning

Crossing, no turns

Crossing turning

Right turn against

Pedes-trians crossing

Traffic signals

Roundabout

Priority crossroads

Priority T&Y intersections

Key Less than 5% 5 to 14% 15 to 24% 25 to 34% 35% or more

Main crash types - rural

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CRASH TYPE

Head On Off road straight

Off Road bend

Rear end turning

Crossing, no turns

Crossing turning

Right turn against

Pedes-trians crossing

Traffic signals

Roundabout

Priority crossroads

Priority T&Y intersections

Key

Less than 5% 5 to 14% 15 to 24% 25 to 34% 35% or more

What is a HRI? – collective risk

High-risk intersections are intersections with a higher than normal risk that someone will die or be seriously injured in the future.

Using recent F&S history is like chasing lightning strikes. To get a better picture: more years of dataUse all injury data to estimate DSIs – gives better prediction. Because of small crash numbers at any one intersection,

a high-risk intersection is only tentatively defined in a short list, and needs to be confirmed by further analysis:

Risks are measured using estimated DSIs, calculated as: • Collective risk: estimated number of DSI equivalents • Threshold 1.1 estimated DSI equivalents in 5 years. 700 intersections – 2.5% of intersections high and medium high

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Personal risk

Where there are enough crashes (4) to permit a valid calculation, personal risk – or risk of DSI per vehicle using the intersection can be assessed. We have also set thresholds for these.

We cannot perform this analysis globally at present because we do not have automatic access to the traffic flow data, but there is now a database with 25,000 intersections in GIS.

We have used this data to set thresholds for high personal risk.

Similar number to high and medium high collective risk with little overlap.

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What is a high risk intersection?

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Treatment Philosophy Strategy

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LoSS and Transformation potential

Once we have the flow data we can also use crash prediction models to compare the reported crashes with those that we would normally expect for an intersection of that type with that amount of traffic. We call this the Level of Safety Service (LoSS).

We can use a similar method to compare the potential for improvement by changing to a different form or control type.

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Level of Safety Service: Urban 4 leg signals

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What is a high risk intersection? - Auckland

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Comparing intersection types

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Comparing intersection types

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Comparing intersection types

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Countermeasures – What have we learnt?

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Pedestrians:

Where pedestrians cross a traffic stream – vulnerable due to traffic speed.Rarely a red running issue - pedestrian compliance - longer cycle times worsen pedestrian safety. Pedestrian optimised? Where traffic is turning, typically speeds are slower – but still severe if a heavy vehicle is involved turning at lights.Half of urban pedestrian and cyclist deaths involve a heavy vehicle.Pedestrians early start?

Countermeasures – What have we learnt?

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Rural intersections:

Priority cross roads and T-junctions are by far the worst – very high risk and high severity.

Roundabouts best - if exits clear zoned.

Rural signals need careful design, phasing, detection and clear zones - but can be lower risk –especially at T-junctions .

Countermeasures – clear zones – at intersections

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How are we responding to the challenge?

The focus on deaths and serious injuriesUsing F&S crashes or estimating DSIs using severity ratios

Identifying high risk intersections. Using injury crash data and estimating DSIs from typical severities. No risk assessment tools equivalent to KAT

Crash types that result in deaths and serious injuriesMore focus on severe crash types e.g. side impacts, pedestrians and cyclists.

Considering how countermeasures affect crash severityRoundabouts, barriers and clear zones

Crash prediction models and crash severity / casualtiesStill requires development – using typical severities.

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Developing the forward SH programme

Strategic assessment – SafetyNET – now includes intersections

Safety works prioritised on DSIs saved / project life / $100m

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Red Light Running – about one third

Failed to notice red signal• Brightness • Location, location, location

• Background distraction / confusion• Progression expectation• Inconsistent phasing - arrows

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Right turn against (LB) – nearly one third

• geometric - visibility • judging speed, intentions - multilane• turning on yellow - one lane stops • through or turning left?• phasing & arrows

Right turn arrows

Reduction in LB crashes due to arrows: (Hall TRRL)

• filter then lag right turn arrow:30%

• lead right turn arrow then filter: 68%• lead right turn arrow (no filter):

90%• Problem with lag turns – amber trap

Countermeasures – What have we learnt?

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Turn lanes mixed blessing

Right turn lanes - reduce low severity rear ends but if not aligned well (typical), are visibility block – increase risk. Also more lanes speed up through vehicles into turning traffic.So may increase high severity crashes while reduce low severity type. While shared left / through lanes not as bad since GW rule changes – (indicating left but travel through) but still much worse for cyclists. Right turn against – mostly a multilane issue.

Pedestrian Conflicts - crossroads

One fifth to one quarter – higher in CBD

InjuryF&S

Ped Crossing from left 13636

· Ped crossing from right 77 16

· Car turns left ped from left 4211 ped from right 19

3· Car turns right ped from left 115

24 ped from right. 77

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Pedestrian Conflicts: through vehicles

Rarely blackspots but some observations

· Mid-block signals safe· Mostly pedestrian violations · Some red light running

Pedestrian Conflicts: turning vehicles

Rarely blackspots but some observations

· Wide intersections intimidate· Distraction by vehicles

· Slip lanes are safe (high entry angle – tight radius)

· Lane layouts that simplify turning decisions · GW rule change should help

Addressing the design of the system

Einstein:

“It is not possible to solve problems using the same kind of thinking that was used to create them.”

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Thank you