Tunnel lighting and ventilation: importance and methods

Lighting: importance and methods

In daytime, eyes adapted to bright outdoor light cannot see into the dark tunnel entrance (the "black-hole" effect); at night, the change between the lit tunnel and the dark road outside also affects drivers. Tunnel lighting is therefore provided in zones.

Importance

• Enables drivers to see the road, obstacles and signs, and reduces accidents.
• Provides gradual eye adaptation between the outside and inside luminance.
• Gives comfort, safety and a sense of direction, and helps rescue and maintenance work.
• Provides visibility during emergencies and evacuation.

Methods/zones (CIE 88 / IRC:SP:91 approach)

1. Access zone: outside the portal, where the driver sees the tunnel entrance; its luminance (L20) governs the entrance lighting.
2. Threshold zone: the first part of the tunnel, with the highest luminance (length about one stopping sight distance); its luminance is a fraction of the access-zone luminance, about 5-10 percent of L20 (factor k = 0.05-0.10).
3. Transition zone: the luminance is gradually reduced (stepwise or smoothly) to the interior level so that the eyes can adapt.
4. Interior zone: a constant lower level depending on speed and traffic.
5. Exit zone: higher luminance near the exit to help drivers adapt to outdoor light; at night the tunnel lighting is reduced instead.

Lighting uses high-pressure sodium or LED luminaires in symmetric, counter-beam (against traffic) or pro-beam arrangements, with a separate emergency lighting supply.

Ventilation: importance

Ventilation is important in a tunnel because:

• It dilutes carbon monoxide, NOx, hydrocarbons and diesel smoke from vehicles to safe levels, protecting the health of users and workers.
• It maintains visibility by removing smoke and dust, which is essential for safe driving.
• During construction it clears blasting fumes, dust and gases (methane, H2S, CO2), supplies oxygen for workers and machines, and controls heat and humidity.
• It controls temperature and humidity, avoiding condensation and corrosion of equipment.
• In case of fire it controls the spread of smoke and heat and keeps escape routes clear.
• It keeps air quality at the portals and in the surroundings within environmental limits.

Ventilation: methods

Vehicles in a tunnel emit CO, NOx, hydrocarbons and smoke, and blasting and diesel plant add dust and fumes during construction. Ventilation supplies fresh air and dilutes or removes pollutants, keeping visibility and air quality within limits (IRC:SP:91 and PIARC give limits, e.g. CO of about 70-100 ppm for normal traffic, and visibility limits for smoke).

Natural ventilation

• Relies on the piston effect of moving vehicles and on wind and temperature differences between the portals.
• Suitable only for short tunnels (below about 300-500 m) with low traffic; not dependable.

Mechanical (artificial) ventilation

1. Longitudinal system: jet fans hung from the roof push air along the tunnel from one portal to the other. Simple and economical; suitable for one-way tunnels of moderate length.
2. Semi-transverse system: fresh air is supplied through a duct along the tunnel and escapes through the portals (or the reverse: an exhaust duct, with fresh air entering at the portals). Used for medium to long tunnels.
3. Fully transverse system: separate supply and exhaust ducts; fresh air is supplied uniformly along the length and polluted air is extracted. Gives the best control but is costly; used for long tunnels with heavy traffic, and also helps smoke control during a fire.
4. Shaft ventilation: vertical or inclined shafts with fans in long tunnels, to remove exhaust at intermediate points.
5. Ventilation during construction: forced (blowing) or exhaust ventilation with flexible ducts and fans, often as a combined (push-pull) system.

 Longitudinal:  ->J- J- J- J->   (jet fans, air flows end to end)
 Semi-transverse: [supply duct] ==> air leaves at portals
 Transverse:    [supply duct]
                 --- tunnel ---
                [exhaust duct]

The choice depends on length, traffic volume, gradient, number of lanes and cost; CO and visibility sensors control fan operation.