Download - 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.
The art of the crash investigator
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 art of the crash investigator
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 art of the crash investigator
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.
The art of the crash investigator
Conflict points at intersections
24 points of major conflict
The fewer conflict
points – the safer!
Conflict points at intersections
6 points of major conflict
The fewer conflict
points – the safer!
Conflict points at intersections
4 points of major conflict
The fewer conflict
points – the safer!
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.
The art of the crash investigator
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
--------------------------------------------------------
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
------------------------------------------------------------
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
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?
The art of the crash investigator
The art of the crash investigator
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
The art of the crash investigator
Always look for visual deceit at any site
Traffic signals
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….