III.1. General considerations

The choice of geometric characteristics shall be such as to afford to all users proper safety and traffic flow conditions with a minimum of congestion, bearing in mind the function of the road and the general behaviour of drivers.

The general rules of design apply to both the construction of new roads and the modernization of the existing network. In the latter case, however, account shall be taken of special constraints and situations and the basic rules shall be applied flexibly so as to conserve the general consistency of the route. Less importance may therefore be attached to some basic parameters while upgrading the quality of the alignment and its perception by the driver (‘readability’ of the road) so as to improve safety.

The progressive improvements to a road shall be effected with particular care so as, at each stage, to respect the general consistency of the route (importance of transitions).

When a motorway or a road with separate carriageways is constructed in stages, involving the initial inauguration of a single two-way carriageway, care shall be taken in designing this first phase so that its two-way nature is clearly recognizable by users and so that it can function as such; this will involve the need to ensure overtaking visibility for traffic in each direction along most of the alignment and, as far as possible, to conceal such installations as must be constructed in their final form from the outset.

The parameters of the design and dimensions depend on the choice of category of road, which is conditioned by its functions, its location (topography, land use, etc.) and the general technical and economic context. The choice of category shall take account of:

Internal consistency (homogeneity) of construction characteristics;

Consistency of the road with the user’s perception of it.

It will then be possible to define a consistent overall approach to the development of the route (or section) under consideration, and to decide accordingly on all the components of the project (geometry, signs and equipment, and junctions).

A range of design speed is associated with each category of road.

The design speed is that speed which in a scheme for the improvement or construction of a road is chosen to determine geometric characteristics permitting isolated vehicles to travel at this speed in safety.

The range of recommended design speeds in km/h on international roads is as follows:

Motorways | X | 80 | 100 | 120 | 140

Express roads | 60 | 80 | 100 | 120 | X

Ordinary roads | 60 | 80 | 100 | X | X

Design speeds of over 100 km/h should not be selected unless the carriageways are separated and the layout of intersections so permits.

The lowest design speeds (60 km/h for roads or 80 km/h for motorways) may be used on highly restrictive sections.

The design speed may be reduced in exceptional cases on sections of limited length of the road and in difficult topographic and other conditions. Changes from one design speed to another should be applied gradually in such a manner that they can be easily foreseen by the driver.

The concept of ‘design speed’ may not be applicable to certain routes with a difficult topography.

International roads shall present homogeneous characteristics over sufficiently long sections. Changes of category shall be made at points where they are quite clear to users (approaching built-up areas, change in topography, interchanges, toll areas and frontier posts.) and particular attention shall be paid to transition zones.

It is also important to verify that minimum conditions of safety are observed at all points on the road, taking into account the actual speeds at which most users travel, in the light of the general configuration of the alignment and the regulations in force.

International roads should provide for traffic of motor vehicles in accordance with national regulations concerning the sizes, total weight and axle load.

III.2. Horizontal and vertical alignment

III.2.1. Basic parameters

The horizontal and vertical alignment shall be co-ordinated in such a way that the road is perceived by the driver as being without undue discontinuities of alignment, permits him to anticipate his manoeuvres and to see clearly the critical points, in particular junctions and entrances and exits of interchanges.

The rules for the dimensions of the horizontal and vertical alignment shall be based on conventional traffic engineering parameters (reaction times, friction coefficients, height of obstacle, etc.) for the majority of users.

The recommended minimum values for the parameters of the horizontal and vertical alignment are given in the following table:

Design speed (km/h) | 60 | 80 | 100 | 120 | 140

Minimum radii in plane (corresponding to maximum superelevation 7%) | 120 | 240 | 450 | 650 | 1 000

Maximum gradient (percentage not to be exceeded)1) | 8 | 7 | 6 | 5 | 4

Maximum longitudinal gradient in new tunnels2) | 5 | 5 | 5 | 5 | 5

Minimum radii at the highest point of the vertical alignment (in m) | One-way | 1 500 | 3 000 | 6 000 | 10 000 | 18 000

Two-way | 1 600 | 4 500 | 10 000 | – | –

Minimum radii at the lowest point of the vertical alignment | 1 500 | 2 000 | 3 000 | 4 200 | 6 000

1)The maximum gradient should be decreased by 1% in the case of express roads and motorways. When the maximum gradient is applied, an additional lane for slow-moving vehicles should be envisaged.

2)Unless no other solution is geographically possible. In tunnels with gradients higher than 3%, additional and/or reinforced measures should be taken to enhance safety on the basis of a risk analysis.

The minimum vertical alignment radii shall be avoided at the approaches to critical points (junctions, interchanges, accesses, entries to built-up areas, etc.).

The gradient resulting from longitudinal slope and superelevation shall not exceed 10 per cent.

Horizontal alignment curves shall, when appropriate, be introduced by transition curves.

III.2.2. Conditions of visibility

Visibility distances shall be at least equal to stopping distances for obstacles over the whole length of the road.

Minimum values are given for guidance in the table below:

Design speed (km/h) | 60 | 80 | 100 | 120 | 140

Minimum stopping distance (m) | 70 | 100 | 150 | 200 | 300

On two-way roads, the minimum visibility distances required for overtaking shall be provided on as great a percentage of the length of the road and be as uniformly distributed as possible.

Where visibility is insufficient, the construction, on single carriageway two-way roads, of passing areas or judiciously-sited local widening of the road is recommended.

In areas where visibility distances cannot be ensured (permanently or temporarily), appropriate road markings and signs shall prohibit overtaking in a form clear and perceptible to users.

III.3. Cross-section between junctions

The formation of international roads shall comprise, in addition to the carriageway or carriageways, verges and possibly a central reserve and special paths for pedestrians and cyclists. Such special paths shall not be permitted within the formation of motorways and express roads.

The cross-section shall be such as to ensure at all times the smooth flow of current and foreseeable traffic in proper conditions of safety and comfort.

In this respect, tunnels and bridges, structures which are an integral part of the road system, should, to the extent possible, with the exception of the emergency lane, have the same number of traffic lanes as there are before and after these structures. Any change in the number of lanes should occur at a sufficient distance from the entrance to these infrastructures.

For tunnels, the principal criteria to be taken into account in deciding on the number of tubes to build (a single tube or two tubes) are traffic forecasts and safety (taking into account such aspects as the percentage of heavy goods vehicles, gradient and length).