To facilitate and clarify communication and comparison it is necessary to define a minimal set of terms regarding carriageway and off-carriageways. The working group which produced the Technical Report 05.11. B. "Cross section geometry in uni-directionnal tunnels" decided to apply the following terminology:
The distinction is justified in that there appears to be general agreement about the use and dimensions of the carriageway, while the dimensions of and requirements for elements of the off-carriageway differ greatly between countries. The emergency lane is defined as an "area of hard clearance to park vehicles in case of emergency".
On roads of the motorway type in the open air usually an emergency lane is provided. Hard clearances in tunnels are often restricted for economic reasons. This restriction can make it impossible for broken-down vehicles to park on the hard clearance adjacent to the driving lane without occupying part of the driving lane and thus disrupting traffic flow.
The geometry of off-carriageways varies between different countries, e.g. no general rules or figures can be given. In many countries, due to costs, the width of the hard clearance is too small to park a vehicle adequately. Therefore at certain distances lay-bys are provided. However in Norwegian and Spanish experience only 40 % of the broken down vehicles effectively reach or use the lay-bys. This demonstrates that lay-bys cannot completely replace emergency lanes: see Sections 8 to 10 of Chapter III "Breakdowns" of Report 05.04.B.
The danger that lay-bys and other lateral obstacles may present to tunnel users has been studied in Technical Report 2016R16EN entitled “Lay-bys and protection against lateral obstacles: current practices in Europe”. This report presents the results of a survey undertaken in several European countries in order to obtain information on the number and layout of lay-bys, feedback on accidents involving lay-bys or other lateral obstacles, reference to any studies conducted on this issue and other relevant information. Initiatives undertaken to improve user safety with regard to lateral obstacles are outlined and the general conclusions of the work undertaken are presented.
The hard clearance should give the possibility to park a stranded car outside the carriageway. Therefore the width measured from the outer side of the edge lane marking should be at least the width of a passenger car (1.75 m) plus a width of 0.50 m. to enable motorists to descend, resulting in a hard clearance of 2.45 m.
In case also heavy trucks should be parked outside the carriageway a width of (2.50 + 0.50 + 0.20 =) 3.20 m is required as explained in Chapter 6 "The off-carriageway" of the Report 05.11.B.
Safety barriers are commonly referred to as "massive construction to guide vehicles colliding with the tunnel side-wall safely back in the direction of traffic". It differs from guard rails, which are a flexible or frangible beam type construction supported on poles to prevent vehicles colliding with the tunnel side-wall.In the case of tunnels it is questionable whether object distance is determined by the distance between the inner side of the edge lane marking and the kerb of walkways, the front of safety barriers or guide rails, or the tunnel side-wall. There is general agreement that in case low level walkways are employed the distance to the tunnel wall is a good measure. When no walkways are present the distance to the base or to the top level of the safety barriers has to be considered.
Especially in tunnels drivers prefer a certain distance to the wall (or walkway, guide rail or safety barrier) due to smaller movements of the eye-angle when fixed on objects. Experience shows that where object distance in tunnels is smaller than on the adjoining road motorists change course to keep distance from the tunnel wall: see Chapter 6 "The off-carriageway" of the Report 05.11.B.
If vehicles crossing the edge lane marking cannot be redirected in time then the consequences of collision with the wall must be minimized. This can be achieved by means of safety barriers or guard rails. Safety barriers require less space than guard rails. When vehicles collide with safety barriers at small (acute) angles they can be guided back in the direction of traffic and there is a chance of preventing major accidents. When vehicles collide with safety barriers at large (obtuse) angles the results of the collision may be more serious. Guard rails are not as effective as safety barriers at correcting/redirecting errant vehicles; however, they cause less damage in a collision at an obtuse angle. That is why safety barriers are to be preferred in case of narrow hard clearances and guard rails in case of broad hard clearances.
As guard rails require bending space this would mean extra width of the tunnel, which in many cases is not feasible from an economic point of view. Especially at restricted speed, safety barriers perform well. Moreover, barriers need less maintenance.