What’s the safe system approach to road design?

Humans are squishy, error-prone and inconsistent and this makes for high risks when driving. We lack the ability to concentrate for long periods of time, our eyesight is much duller than that of a bird of prey and our perception of rapidly changing objects is far inferior to that of chimpanzees. In order to compensate for our inadequacies when driving, we have to use a ‘safe system’ of road design to limit the carnage.

The core concept of the safe system is that humans do make mistakes on the road, but a mistake should not result in serious injury or death.

The four pillars are:

1. Safer roads
2. Safe speeds
3. Safer vehicles
4. Safer road users

The first line of defence is to help prevent collisions. The second line of defence is to ensure that impact forces from any collision that couldn’t be prevented are reduced to a level well below the maximum human tolerance. 

On rural roads and major arterial roads, crashes tend to involve one or more vehicles and often an immovable object on the side of the road like a tree. In urban areas, pedestrians and cyclists are much more likely to be involved. Pedestrians and cyclists are even more likely to be killed than people in a vehicle or wearing motorbike protective clothing.

Using the safe system, safer roads means designing a road where a road user automatically picks a safe speed and can safely negotiate the curves, contours and obstacles associated with that road without coming into conflict with another road user. If conflict does occur, there is a forgiving roadside environment which allows for margins of error. Any demands placed on the driver don’t exceed the realms of capability of anyone who is likely to be using the road and doesn’t offer up any surprises. 

The safe system can’t guarantee absolute safety: however ‘safe’ you make a road, there will always be someone who will push beyond their limits.

The features of a safe road environment are:

1. Consistent design standards – consistent signage and surfaces, predictable curves, etc
2. Obvious transitions between sections of differing standards – clear markings and indications where the road might become more hazardous or changes its characteristics, for example, urban road to motorway, or using sightlines to make drivers away of upcoming hazards
3. Controlled information release – information given to the driver is done so at a rate they can absorb it and act upon it
4. Repetition – information is repeated where it’s important
5. Suits its designated users – for example, pavements and crossings in urban areas; barriers and slip roads in motorway areas.

A safe road should:

1. Warn users of any substandard or unusual features – for example, a narrow road
2. Warn users of road conditions – for example, slippery when wet
3. Guide users through unusual sections – for example, a chicane
4. Be forgiving of driver error – for example, road shoulders
5. Be self-explaining – for example, obvious road markings and good lines of sight
6. Control road users at conflict points and sections – for example, traffic lights, roundabouts, slip roads

Road design is governed by a set of engineering standards, but using the engineering standards alone does not ensure the road is safe. Applying the five control measures to the engineering standards goes a long way to helping prevent crashes.

Safe design

Safe design takes into consideration vehicle speed and the speed limit, maximum operating characteristics of vehicles, visibility,  road width, corner angle (total angle and whether the radius tightens or widens), traffic characteristics, change in elevation, the presence of junctions and more. The carnage of any impact is directly affected by the speed of the collision and the speed of the collision is a function of how much time a person has to react to the danger.

Choice of junction

A junction is a conflict point. A large percentage of serious injury crashes occur at junctions so constructing these in a way that makes it easier for road users to make the right choice is a high priority. 

The type of junction chosen should be completely obvious to a road user and should be chosen based on how traffic should be controlled. For example, a crossroads on a busy road can be more dangerous than a roundabout, whereas a roundabout on a road where road users may need to slow down dramatically might not be the optimal choice. Road designers can use bridges, traffic lights and other types of junctions to deal with conflict points, depending on whether pedestrians and cyclists will be involved.

The roadside

Solid objects on roadsides provide areas of danger: they cause damage when they are hit and they can block sightlines. Improving road safety means addressing each one of these objects in this order:

1. Removal – does it need to be there at all?
2. Relocation – if it does need to be there, can it be moved further away, e.g. relocating post box further from the kerb or into a side street with less traffic?
3. Redesign so they can be traversed – if it can’t be moved, can it be modified, e.g. ramps in culverts?
4. Redesign so they become frangible or less damaging – if it can’t be traversed, can it be made to break away, e.g. streetlights that snap upon impact?
5. Shielded – if it can’t be made frangible, can barriers be used to absorb and/or deflect energy?
6. Unambiguous warning – if it can’t have barriers, can there be warning signs, e.g. areas that flood periodically, cliffs in close proximity to the road, etc?

Overall considerations

Road designers must also consider community concerns, reducing travel times (ameliorating congestion, for example), reducing freight costs, improving public transport and providing a safe environment for pedestrians and cyclists.