How are roads designed using the safe system approach?

Humans are deeply flawed – far too flawed for safe driving and we see that every day in the form of crashes that result in death and injury. Our reactions are relatively slow, our eyesight is not good at night and we struggle to deal with multiple stimuli at the same time. A combination of vehicle technology and road design makes driving much safer than it otherwise would be. While we’ve covered much of the vehicle technology before (features such as automatic emergency braking and electronic stability control), road design makes up a big portion of the equation.

A safe system acknowledges that humans make mistakes and there are four principles:

1. 1. Safer roads – designing roads so that they are forgiving of errors
   2. Safe speeds – keeping speeds to a level where any mistake is not punished by serious injury or worse
   3. Safer vehicles – mandating technology that is proven to reduce harm
   4. Safer road users – ensuring training and competence.

Preventing collisions is the first priority. Where that fails, minimising the forces the human body experiences is the second priority – by reducing them below the maximum human tolerance, survivability of accidents is increased.

The type and nature of accidents need to be considered. For example, in heavily urbanised areas, crashes between pedestrians and cyclists are more prevalent than on rural roads where accidents tend to involve the driver leaving the road or colliding with another vehicle at an intersection.

Safe road design helps the driver make the right decisions about their driving and allows the driver to receive information about the road’s course and surface at a speed which is easy to assimilate and act on. If the driver does make a mistake, margins are built in to allow for a recovery or the roadside environment is forgiving enough to bring the vehicle to a halt without causing injury to the occupants.

However, absolute safety can’t be guaranteed in a safe system because there are always people who will underestimate their limits.

Safe road environments are designed with five key features:

1. 1. Suitable for its users: urban areas will have more focus on safe pedestrian movement at the expense of vehicle speed, whereas motorways prioritise fast movement of traffic and separation from vehicles approaching in the opposite direction
   2. Information release is controlled: signage and road markings are delivered to the driver in a logical order and at reasonably spaced intervals so that the driver can cope with the information
   3. Important information is repeated: where relevant, information is given more than once to ensure that the driver is aware of an upcoming hazard, for example, a compulsory left turn at an intersection could be signposted in advance, marked on the road and also signposted at the intersection.
   4. Design is consistent: signage and markings are predictable in relation to hazards; road widths suit the speed; bends in the road are predictable
   5. Transitions are obvious: road markings, signage and road characteristics are all varied consistently to indicate changes in the type or frequency of hazards and to help a driver choose an appropriate speed without the need to always be told.

While all roads are built to certain engineering standards, using those standards alone doesn’t necessarily mean the road is safe. Safe roads also:

1. 1. Control road users where they will come into conflict with other road users: for example, traffic lights, pedestrian crossings, direction arrows, etc
   2. Are self-explanatory: for example, signage that consistently directs a user to a point of interest
   3. Are forgiving: for example, turning bays for right turns on busy roads
   4. Guide road users: for example, show the direction and type of road where it might be obscured by foliage
   5. Warn road users: for example, give awareness of hidden features such as a sharp sequence of bends, or where ice might be a problem, or where a feature might be unexpected

Consideration is given to:

• • The speed limit
  • Vehicle speed, e.g. higher speeds need longer warning periods
  • Vehicle capability, e.g. it’s more difficult for a long vehicle to turn a sharp hairpin bend
  • Visibility, e.g. the view ahead is compromised by topological features like a cliff
  • Road width, e.g. road narrows going through a tunnel
  • Corner angle, e.g. sharp enough to require a significant reduction in speed
  • Corner characteristics, e.g. adverse camber, radius tightens
  • Change in elevation, e.g. can a vehicle match the speed of other vehicles as it exits an on-ramp
  • Intersection, e.g. will there be a conflict between turning vehicles and pedestrians

Intersections

Intersections pose a large risk for road users. Crashes at t-intersections are common therefore is it better to have a roundabout or traffic lights to reduce risk?

The reason for and layout of a particular intersection should be obvious to a road user.

The road environment

While the ideal system would see road users remain on the road, this isn’t always going to happen and therefore the environment around the road has to be considered. Danger factors for road users are:

• • Heavy, non-deformable objects on the side of the road, e.g. trees, large rocks
  • Steep drops, e.g. a cliff or bank
  • Street furniture, e.g. large plant pots
  • Canopies, e.g. on shops

• • Vegetation that obscures views

The safety of the road environment is considered in this order:

1. 1. Remove it: does the object even need to be there?
   2. Relocate it: can it be moved somewhere else?
   3. Redesign it to be traversed: can the object be overcome with a redesign?
   4. Redesign it to be frangible: can the object be changed so it is less damaging if hit
   5. Shield it: can a barrier be placed around it
   6. Warn of it: clear signage.

Using this system acknowledges that not all obstacles at the side of the road can be eliminated but the risk of them causing a problem can be mitigated.