Explanation of the CIE88 2004 Tunnel Lighting Standard

The standard is officially called "Guide for the Lighting of Road Tunnels and Underpasses" and this article is about the 2004 version, which is currently in use. It replaces the 1990 version. 

This article is a user friendly summary of the standard and ignores some details, for example daylight screens and emergency lighting.

I'll just be looking mainly at the requirements for lighting tunnels in the daytime, a large part of which concerns the effect of the luminance of the areas surrounding the tunnel entrance.

For an explanation of what luminance means, this book will help you:

You may have noticed that as you approach a long tunnel in bright sunlight the entrance often seems like a black hole: 

 

In these cases, if there was an obstacle (like a stopped car or a drunk pedestrian) just a few meters into the tunnel you would not know it until it you heard the thump!

(Note that you need to know the difference between luminous intensity, illumination and luminance to understand this article.)

The problem with tunnel lighting design is that tunnels have many variables which affect their safety. Some of the most important are:

• The age of the driver.
• The dryness or wetness of the road.
• The speed of the automobile.
• The external light conditions.
• Atmospheric conditions (fog being an extreme example)
• Traffic density.
• Maintenance of the tunnel lighting.
• The inclination of the tunnel.

It is neccessary that the driver sees into the tunnel entrance from the outside so he or she is justifiably confident about entering the tunnel. Of course the other concern is that once inside the tunnel they can understand the geometry of the tunnels easily. i.e they can see the walls and know that they are in the correct lane!

Generally lighting a tunnel at night is easier than lighting a tunnel during the day. If it is dark outside, then the change from driving outside a tunnel and driving inside a tunnel is smaller.

During daylight hours the problem is that while the eye can adapt from light (outside the tunnel) to dark (inside the tunnel), it cannot adapt very quickly. A slow moving car gives the eye more time to adapt than a fast moving car. That's one reason not to exceed the speed limit when entering a tunnel!

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The obstacle

The CIE88 2004 standard has taken the position that it is impossible to define a level of visibility for all sorts of objects you might find inside a tunnel. Some tunnels contain only cars, some tunnels contain pedestrians and cyclists. Some cyclists don't switch on their lights! So the standard takes a "standard object" which must be visible at the entrance to the tunnel. This standard object is is a 0.2m x 0.2m square having a reflectance of 0.2: 

The object or obstacle is placed at the entrance to the tunnel, standing on the road surface, and the "stopping distance" is such that when the driver sees the object he must be able to stop in time without crashing into it.

Long and short tunnels

There are officially defined long and short tunnels, and in general short tunnels are those where the driver can easily see the exit of the tunnel from the entrance. This means that some "short in length" tunnels are treated as long ones when the short tunnel bends and the driver cannot see the exit from the entrance.

So there are three cases

1. Geometrically long tunnels.
2. Optically long tunnels (they may be geometrically short, but they're bent).
3. Short tunnels (short and straight)

The sort of lighting level which needs to be applied is determined by the decision graph shown below. You start at the top answering the questions as you go downwards until you come to one of the three answers:

(I often wonder what we are expected to do if the answer to question 4 is 0.3, i.e. medium wall reflectance.)

The threshold zone lighting level is the lighting level at the start of the tunnel.

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Explanation of terms used in CIE88 2004 standard.

Design speed: The speed for which the tunnel is laid out. It is often the same as the maximum speed allowed just outside the tunnel.

Reference point: A point in the center of the approaching lanes 1.5m above the road surface and outside the tunnel at the stopping distance away.

Stopping distance: The distance neccessary to safely stop the vehicle moving at the design speed. It is composed of the distance taken for the reaction (of the driver) time and the distance taken for the braking time.

Vertical Luminance, Ev: Vertical luminance is simply the luminance of a vertical plane, the normal to the plane is horizontal.

Contrast Revealing Coefficient, qc : The ratio between the luminance of the road surface and the vertical luminance at a given position: qc = Lr/Ev , illustrated here:

Symmetric Lighting: When the luminaire throws light equally backwards against the traffic flow and forwards with the traffic flow. In the example below the photometric solid has "wings" which are symmetrical along the traffic flow.

Counter-beam Lighting (CBL): When the luminaire throws light "backwards" into the flow of the traffic. 

Pro-beam Lighting: When the luminaire throws light along the flow of the traffic. 

The luminance curve for tunnels

A very important graph shows how the the luminance should change as the car moves into, through and out of the tunnel:

In general for long tunnels the interior zone is much longer than shown above. I've contracted the interior zone so the interesting entrance and exit luminances are show clearer. Remember that the above graph is a graph of luminace against distance.

Let's look at a more detailed version of the first half:

Lseq, Lth, Ltr and Lin are all luminances, hence the "L". They are explained in more detail below. Luminance is roughly the apparent brightness, what the eye percieves, not to be confused with illumination or luminous intensity.

Note that the Access Zone and the Stopping Distance (SD) are the same. The access zone is the section of road before the tunnel entrance, starting outside the tunnel, at the stopping distance from the tunnel entrance. So Lseq is the luminance in that section of "open" road. Notice that luminance falls once we get near the tunnel, because the tunnel mouth will start to dominate the visual field. This is shown graphically here:

Consider the three images, as the tunnel gets closer the "average brightness" percieved by the eye goes down. However well lit, in the daytime, the tunnel always has a luminance lower than the external environment.

Once the car is inside the tunnel it is in the "Threshold Zone", called this because the car is on the threshold between external road and the tunnel proper. As shown above the length of the threshold zone should be at least the stopping distance (SD).

Right after the Threshold Zone is the Transition Zone, where the luminance will fall to a (more or less) fixed value which most of the tunnel will have.

The Interior Zone has the fixed luminance value which will last until the car gets to the Exit Zone.  

The Exit Zone is often where external daylight illuminates the last part of the tunnel, and where the driver sees the external, brighter, landscape dominate his or her visual field.

The values Lth etc are generally taken to be minimum, and tunnel lighting should be at these minimums of above them.

Percieved contrast.

The percieved contrast is defined like this:

So it is the relationship between the luminance of the object and the luminance of the road. Obviously we'd like them to be different, if they are the same the contrast is 0! Remember that the object is a 0.2m square with reflectance (rho) of 0.2.

Now Lop and Lrp (used in the equation above) are defined as a sum of other luminances passing through mediums of varying transparency (transmittance). The windshield for example will reduce the luminance of the object because it does not transmit all the light which hits it. And the atmosphere too is not completely transparent. 

Lop and Lrp are calculated like this:

For example the atmosphere between you and the obstacle has a luminance (very small usually, unless you are in brightly lit fog) and this lumiinance is attenuated by tws, the transmittance of the windscreen.

Lseq is important. It is called the Equivalent Veiling Luminance. When light enters your eyeball it bounces around and gives a veil of light over the ordinary clean image. Lseq is considered to come from all the objects around a 2° cone of vision. The driver should be concentrating on that 2° cone, but the veiling luminance will reduce the contrast of what he sees.

It is not explicitly stated in the standard but I assume that the 2° cone of vision goes to the high resolution part of the retina. Other parts of the retina are medium or low resolution.

Compare perceived contrast with intrinsic contrast. The latter is the contrast when the you are very close to the object, in other words when there is no atmospheric or glare effects. Percieved contrast is different from intrinsic contrast because you are far from the object and light from other sources enters your eye, and the atmosphere between you and the object also reduced the contrast.

Lighting in the threshold zone.

You must be able to see other road uses in the dark threshold zone while you are driving outside the tunnel and are at the stopping distance away from the tunnel entrance. Obviously we are trying to avoid the "black hole" effect. Mathematically the percieved contrast should be at, or higher than, a given minimum.

Lth is the luminance in the first part of the tunnel, and is the horizontal section of the threshold zone (after the tunnel entrance in the graphs above). Lth is calculated like this:

Cm is the minimum percieved contrast required percieved contrast required. Rho is the reflectance of the obstacle (often set at 0.2) and qc is the contrast revealing coefficient. Generally all these numbers are given to us, except for Lseq...

So Lseq is the luminance created inside the 2° cone by light outside of the 2° cone. So this surrounding light veils what you are looking at, reducing the contrast.

How is Lseq calculated? You can either actually go to the tunnel and measure it with appropriate instruments, or use a graphical method explained below.

A polar grid (Adrian's diagram) is superimposed on the view of the tunnel entrance and its surroundings. Here is the grid :

You can understand it better if you see it over a photo:

The 2° cone is shown by the inner circle with the X in the middle. Inside your eye light from the other sectors invade that inner disk (on the retina of your eye) and reduces visibility there. The grid helps us get an idea of the luminance surrounding the cone.

Each area has been calculated to have the same influence on the 2° cone as all the others. Larger areas at the edge of vision have the same effect as smaller areas near the center of vision, given the same luminance in both areas.

The image above is actually a screenshot from a program which will sum the areas in the correct portions for you, giving you a value for Lseq. The program is LITESTAR 4D Tunnel Plus from OxyTech. Here is a fuller screenshot:

(For different standards there are slightly different radial grids, click here for a comparison of UNI11095 2003, CIE88 2004 and UNI11095 2011)

The standard requires that each quadrilateral is assigned a percentage of Sky, Road, Rocks, Building, Snow, Vegetation and Tunnel mouth. And each type of area is assigned a luminance.

Different areas occupy different amounts of the "quadrilaterals". Here is an example:

For example Sky is 8 kcd/squ-m in the example shown below. The luminances change with season and hour of course.

Lseq is a weighted sum of all the quadrilateral areas. The weights are the percentages of area type (Sky, Vegetation etc.) present in the area.

Back to that horrid looking formula:

We calculate Lseq with the grid, and all the other values in the equation are known to us. The threshold zone's constant luminance of Lth should last half the stopping distance, and then fall linearly to 40% of Lth. At which point we move into the transition zone...

The transition zone length and luminance.

The transition zone is the last zone before we hit the internal lighting zone of the tunnel. Here is a closeup of the move from threshold zone to transition zone. 

In the transition zone a new formula takes over, as shown above. The numbers are arranged so that at t=0 (0 meters into the transition zone) Ltr is almost exactly 0.4, thus taking over from the 40% linear fall in the second half of the threshold zone.

How do we calculate the transition zone length? Contact me for a detailed explanation.

How do we calculate the stopping distance? Contact me for a detailed explanation.