5 intersection safety

Semarang 23rd & 24th April 2012

Intersection safety

Phillip Jordan, Consultant

Road Safety International Pty Ltd

• To explain why we need to consider the safety of intersections.

• To give some details of how to improve safety at intersections.

Objectives of this presentation

An intersection can be defined as :

“a place where two or more roads meet at grade”.

Intersections are particularly high risk locations because different road users (trucks, buses, cars, pedestrians, and motorcycles) are required to use the same space.

Intersections are critical locations in the road network in terms of capacity, level of service and safety.

They are the place where opposing streams of traffic have to compete for space and time.

They are high risk locations for crashes because road users on conflicting paths in intersections are required to use the same space, and a collision is only avoided if they are separated in time!

The art of the crash investigator

SERIOUS CASUALTY CRASHES – AUSTRALIA 2010

Intersections also present a risk of serious injury or death when a crash occurs because of the potential for high relative impact speeds.

Intersections are the location of up to 50% of reported

urban crashes in most countries. They are the location of between 10-20% of reported

rural crashes in most countries.


Intersections can be found in urban and rural areas. They are usually in one of four main groups:

– Four way (cross road) intersections

– T intersections

– Y intersections

– Multi-leg intersections


The main forms of control at intersections are: – Road Rules (no physical control) - relying on a priority rule to

indicate right of way;

– Priority road designated by ‘Give Way’ or ‘Stop’ signs;

– Roundabout;

– Traffic signals, with partial control of right turning traffic;

– Traffic signals, with full right turn control;


The basic principles you need to consider and try to provide for safe intersections are:

• Reduction in the relative speed between vehicles • Give precedence to major traffic movements; • Separate conflicts in space and time; • Define and minimize conflict areas; • Define the vehicle paths; • Provide clear indications of right-of-way requirements; • Provide for all vehicular and non-vehicular traffic likely to

use the intersection; • Simplify the driving task; • Minimize road user delay.


Conflict points at intersections

24 points of major conflict

The fewer conflict

points – the safer!



The fewer conflict


ASD is the minimum level of sight distance which should be provided at an intersection. It is equal to car stopping sight distance (SSD) – which is defined as the distance travelled by a vehicle between the time when the driver receives a stimulus indicating a need to stop, and the time that the vehicle does come to a stop.

This distance is sufficient to allow drivers/riders to be able to see the line marking and kerbing at the intersection.


Basic elements of intersection safety

• ASD – approach sight distance

• SISD – safe intersection sight distance

• ASD addresses overshoot crashes

• SISD addresses restart problems

With intersection right angle crashes – we need

to determine if the crash is an overshoot or a

re-start

Why?

Because our countermeasure(s) may be quite

different

With intersection right angle crashes – we need

to determine if the crash is an overshoot or a

re-start

Overshoot – the driver did not know the intersection

was there

Re-start – knew it was there, slowed, maybe

stopped, but selected a “wrong” gap

Overshoot – the driver did not know the

intersection was there

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Improve Approach Sight Distance

Make intersection more conspicuous

Advance warning signs

Advanced direction signs

Duplicate GW or Stops

Lighting

Roundabout or signals

Re-start – knew intersection was there,

slowed, maybe stopped, but selected a

“wrong” gap

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Improve Safe Intersection Sight Distance

Maximise sight lines

Reduce speeds,

Alter the traffic control

Geometric changes

Cut trees/grass

Reduce speed limits

Roundabouts or signals

Y junctions are very dangerous

We need to eliminate Y junctions

Treatment of low angle intersections

Modified Treatment

•Reinforces priority for increased safety

• Enhances sight lines

•Controls turn speed

Treatment of low angle intersections

This is not the way to safely manage traffic for the future. Keep intersections open and provide correct traffic control

Roundabouts are the safest form of intersection control. Why?



Why are roundabouts “safe”? • Few conflict points (only four) • Good geometry induces slow entry speeds • Clearly defined “right of way” • Simple decision making process for drivers/riders • Low relative impact speed if collisions occur

Always look for visual deceit at any site

Traffic signals

Vehicle actuated traffic signals are best. -They reduce lost time. -They usually have “in ground” detector loops that record the presence of a vehicle and tell the controller to give that approach a green signal. - Much more efficient than fixed time signals – and also much safer

Summary

Road safety engineering is a profession that can greatly reduce road crashes at intersections

I look forward to your questions….

5 intersection safety

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