Denmark: Gladsaxe replaces white street lighting with red lighting for bats

In Denmark, one municipality has chosen to swap the white light from its street lamps for red lighting. The aim is to shield nocturnal wildlife, which is highly vulnerable to light pollution, while also cutting the city’s energy use.

Since 8 February, drivers in Gladsaxe, a small municipality on the outskirts of Copenhagen, have seen Frederiksborgvej-one of the main roads leading out from the Danish capital’s centre-take on a vivid red appearance. The former street lamps lining it have been replaced so that they no longer cover the road with the bluish-white glow typical of LEDs, now widespread in European cities.

Although white light is favoured by most major cities across Europe primarily for road-safety reasons, this light spectrum creates substantial light pollution for certain nocturnal animals. It is a post-anthropocentric urban experiment in which the city adjusts to the animal kingdom, rather than the other way round.

A wall of light: bats’ nightmare

The road runs alongside dense woodland inhabited by two particularly vulnerable bat species: the common pipistrelle (Pipistrellus pipistrellus) and the brown long-eared bat (Plecotus auritus). These flying mammals are severely disrupted by white LED lighting.

Bat eyes are highly responsive to short wavelengths-blue and ultraviolet light-and white LEDs, which contain a high proportion of blue light, overwhelm their retinal receptors. For these species, crossing this curtain of light means risking exposure to natural predators such as the common kestrel and the long-eared owl.

While the land surrounding Frederiksborgvej is physically open, it is biologically closed, as bat colonies become confined to either side of the road. Unable to reach their roosts or feeding areas, the landscape loses its ecological permeability, trapping them in vegetation corridors from which escape is extremely difficult.

Red lighting on Frederiksborgvej: a truce between tarmac and woodland

In standard white LED lighting, whether used in street lamps or other fixtures, a blue-emitting diode-at roughly 450 nm-is used to activate a phosphor layer that then generates the other colours. This blue component is what causes bats’ light aversion.

Gladsaxe therefore opted to replace the old lamps along Frederiksborgvej with new units fitted with monochromatic red LEDs. Their output is concentrated solely at wavelengths above 620 nm. As bats lack photoreceptors sensitive to long wavelengths, they barely detect this light and can finally cross the road whenever they need to.

Red LEDs protect bats without compromising road safety

Human vision, by contrast, detects red very effectively because the retina contains L cones specifically adapted to long wavelengths. Motorists can therefore drive safely, even though bats flying above the area perceive the space as completely dark.

This urban scheme forms part of Lighting Metropolis – Green Mobility, a European cross-border cooperation programme covering the Greater Copenhagen region, including eastern Denmark and southern Sweden, in Scania. It seeks to turn the region into a global leader in intelligent lighting and sustainable mobility through innovative LED use that improves urban safety and saves energy.

What, then, are decision-makers in France waiting for before bringing this type of idea to the country? Replacing public lighting-now largely outdated across much of the country-is insignificant compared with the services bats provide us. By helping to curb mosquito populations free of charge, among other benefits, these unfairly disliked mammals protect our health and surely deserve cities adapted to their way of life.