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Intersection safetySemarang23rd & 24th April 2012 Phillip Jordan, Consultant Road Safety International Pty Ltd
Objectives of this presentation• To explain why we need to consider the safety of intersections.• To give some details of how to improve safety at intersections.
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 thesame space.
Intersections are critical locations in the road network interms of capacity, level of service and safety.They are the place where opposing streams of traffic have tocompete for space and time.They are high risk locations for crashes because road users onconflicting paths in intersections are required to use the samespace, and a collision is only avoided if they are separated intime! The art of the crash investigator
Intersections also present a risk of serious injury ordeath when a crash occurs because of the potential forhigh relative impact speeds.Intersections are the location of up to 50% of reportedurban crashes in most countries.They are the location of between 10-20% of reportedrural crashes in most countries. The art of the crash investigator
Intersections can be found in urban and ruralareas. They are usually in one of four maingroups:– 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 The fewer conflict points – the safer! 24 points of major conflict
Conflict points at intersections The fewer conflict points – the safer!6 points of major conflict
Conflict points at intersections The fewer conflict points – the safer! 4 points of major conflict
ASD is the minimum level of sight distance whichshould be provided at an intersection. It is equalto car stopping sight distance (SSD) – which isdefined as the distance travelled by a vehiclebetween the time when the driver receives astimulus indicating a need to stop, and the timethat the vehicle does come to a stop.This distance is sufficient to allow drivers/riders tobe able to see the line marking and kerbing at theintersection. The art of the crash investigator
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-startOvershoot – 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
This is not the way to safelymanage traffic for the future.Keep intersections open andprovide correct traffic control
Roundabouts are the safest form of intersection control. Why? 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
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