During the holiday getaway period, traffic increases at junctions and access roads that many drivers are unfamiliar with. A late instruction from a satnav, an exit spotted at the last moment or a sudden reduction in speed can lead to abrupt changes in direction.

These manoeuvres are particularly critical at diverging junctions, where the main carriageway separates from an exit slip road. This creates a triangular area, commonly known as the gore, with a nose at which barriers, parapets, changes in level and other rigid features may converge.

Protecting this area requires more than simply extending the barriers along both routes. The design must assess possible approach paths, the available space, traffic speeds and how the different vehicle restraint systems connect to one another.

Why is a diverging junction a particularly sensitive location?

On a conventional road, the main route is usually clearly defined. At a diverging junction, drivers must interpret the signs, choose a route and adjust their position within a limited space.

When this decision is made too late, it can result in:

  • Sudden braking before the slip road.
  • Abrupt lane changes.
  • Vehicles entering the hatched area.
  • Impacts with the leading end of a barrier.
  • Head-on collisions with the nose of the gore.
  • Loss of control between the main carriageway and the exit.

The technical challenge arises when the barriers protecting both routes converge at a single point. Although these systems are designed to contain lateral impacts, the end of a barrier or parapet is not necessarily designed to withstand a head-on collision.

The solution must protect this point without introducing a new rigid obstacle into the vehicle’s likely path.

A conventional barrier cannot protect the nose of a diverging junction

Safety barriers and parapets are longitudinal systems. Their performance is primarily assessed for vehicles striking them laterally, which must be contained and redirected towards the carriageway.

At a diverging junction, however, a vehicle may approach head-on or at an angle very different from that envisaged for the longitudinal section. Directly joining the barriers along the two routes can therefore leave a highly rigid point exposed to traffic.

Crash cushions are specifically designed to protect obstacles that may be struck head-on. Their deformable structure progressively absorbs part of the collision energy and reduces the deceleration transmitted to the occupants.

They are commonly used at:

  • The noses of forks and diverging junctions.
  • The leading ends of central reserves.
  • Structural piers.
  • Toll plaza approaches.
  • The leading ends of rigid parapets.
  • Obstacles that cannot be removed or relocated.

Crash cushion or barrier terminal?

Crash cushions and terminals may look similar, but they do not perform exactly the same function.

A barrier terminal provides an appropriate end treatment for a longitudinal system. It connects to the end of a barrier or parapet and is intended to prevent that end from presenting a hazard to vehicles approaching head-on or from the side.

A crash cushion, by contrast, is positioned in front of an obstacle or a particularly hazardous arrangement. It does more than terminate a barrier: it acts as a separate device with a substantial capacity for deformation.

The choice between the two must be based on the actual site layout. At a straightforward diverging junction, where the barriers are sufficiently separated and set back, suitable terminals may be considered. Where the barriers converge or there is a rigid obstacle at the nose, a crash cushion may be required.

Metalesa explores this distinction in greater detail in its article on the criteria for using crash cushions and terminals.

Factors influencing the choice

There is no universal solution for every diverging junction. The design must consider several factors together.

Speed and performance level

Crash cushions are classified according to the speed used in their impact tests. The range may include performance levels 50, 80, 100 and 110, corresponding to the test conditions established for each device.

Selection must take account of the road speed and the path that an out-of-control vehicle could follow. On high-speed roads, both the impact energy and the distance required to absorb it will be greater.

Obstacle width

The device must cover the feature it is intended to protect. The narrow end of a barrier does not require the same configuration as two converging parapets or a wide central reserve.

Metalesa’s range includes crash cushions with different widths and performance levels, allowing the system to be adapted to different road layouts.

Deformation space

A crash cushion needs space to deform during an impact. Its position must prevent interference with obstacles behind it, changes in level, foundations or other features that could prevent it from functioning as intended.

The lateral space required and any potential encroachment into traffic lanes or hard shoulders during deformation must also be assessed.

Approach paths

A vehicle may reach the system from the main carriageway, from the exit slip road or after partially entering the hatched area.

It is therefore insufficient to assess only a fully head-on impact. The orientation, width and position of the device must account for the different foreseeable approach directions.

Connections to the barriers

When a device is integrated between longitudinal systems, continuity at the connections is essential. A poorly designed connection can introduce abrupt changes in stiffness, snagging points or unprotected areas.

The design must check compatibility with the barriers along both routes and ensure that no ends, posts or rigid components remain exposed.

Positioning within the hatched area

One of the most complex considerations is deciding where the device should begin within the diverging junction.

Installing it too close to the nose can leave part of the barriers exposed to a head-on impact. Positioning it too far upstream can occupy an area that could otherwise be traversed or bring it too close to the traffic lanes.

It is also necessary to determine which hard shoulder takes priority: the one alongside the main carriageway, the one alongside the exit slip road or a compromise between the two.

The Spanish Circular Order 35/2014 includes references to the use of crash cushions and terminals at diverging junctions. However, Metalesa notes that some aspects of positioning and selection are not addressed in the same detail as the criteria applicable to barriers and parapets. These include occupation of the hatched area, clearance from hard shoulders and the choice between one or several devices.

Each site therefore requires an individual assessment, coordinating the geometric design with the performance of the selected system.

The importance of full-scale testing

Crash cushions are assessed in accordance with UNE-EN 1317-3. The tests cover different speeds, directions and approach angles to assess their energy absorption capacity and the vehicle’s behaviour during the collision.

The performance characteristics assessed include:

  • Impact severity for the occupants.
  • Lateral displacement of the device.
  • Redirection capability.
  • The vehicle’s path after the collision.
  • Longitudinal deformation of the system.

For example, Metalesa’s C110 models are tested to performance level 110 and carry CE marking in accordance with the applicable requirements.

These characteristics must be considered alongside the actual project conditions, as the outcome depends on both the product and its positioning, installation and connections to the surrounding equipment.

Installation, inspection and replacement

Once the system has been selected, installation must follow the assessed configuration, including foundations, anchorages, orientation, height, components and distances from the barriers.

After an impact, the device must be inspected to determine which parts require replacement. The connections, anchorages and nearby longitudinal systems must also be checked.

At access roads with high seasonal traffic volumes, maintenance before peak travel periods provides an opportunity to check that the crash cushion retains its geometry, that no components are deformed and that its operating area remains clear of obstacles.

Providing protection when a driver makes a late decision

At a diverging junction, a wrong decision can turn a normal vehicle path into a head-on collision within seconds. Advance signing and a clear road layout help prevent this situation, but passive safety measures must be ready to act once an error has occurred.

Terminals provide safe end treatments for barriers. Crash cushions protect noses, obstacles and layouts that may be struck head-on. Connections and transitions ensure that the overall system maintains the necessary continuity.

Metalesa designs and supplies road equipment to address these layouts, with a range of crash cushions, terminals and vehicle restraint systems suited to different speeds, widths and geometries.

On access roads to tourist destinations, increased traffic makes the risk more apparent. However, the need to protect diverging junctions correctly remains throughout the year: a safe junction must be designed to prevent a late decision from becoming a collision with irreversible consequences.