Lighting design that protects bat commuting routes

Bat friendly lighting design starts with the retained habitat feature, not with the luminaire schedule. A hedgerow, treeline, ditch or river edge that bats use to move between a roost and their foraging grounds only works if it stays dark, and the practical question on most sites is how much light falls on that feature at the height the animals fly. Everything else, colour temperature, optics, controls, follows from the answer.

Joint guidance from the Bat Conservation Trust and the Institution of Lighting Professionals is the document most ecologists and lighting designers work to, and reviewers should expect it to be cited by note number and year.

Why lighting changes bat behaviour

Light affects bats in two ways that pull in opposite directions, which is why a simple statement that lighting is harmful does not survive scrutiny.

Light averse species avoid illuminated areas. Lesser horseshoe, greater horseshoe, Myotis species and brown long-eared bats will abandon a commuting route that becomes lit, and if the alternative route is longer or exposed, the cost falls on the colony rather than on the individual. A lit roost exit can delay or prevent emergence altogether.

Other species, particularly pipistrelles and noctules, forage opportunistically around lights where insects accumulate. That looks like a benefit and is not, because it concentrates activity and draws bats away from the wider resource, while the insects themselves are being removed from a landscape that other species depend on.

So the assessment has to be species specific, and it has to rest on survey data rather than on a generic paragraph. Where roosts or commuting routes are involved, that survey work requires a licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017, and a Protected Species Survey drafted for an application is a starting point for that person rather than a substitute.

Setting a target at the retained feature

The design target is a light level at the feature, expressed at the relevant height, not an average across the site.

Guidance points to keeping illuminance on retained bat habitat as low as practicable. A benchmark of 1 lux at the feature is commonly applied, with a lower figure sought where light averse species are present, and the number for a particular scheme should come from the project ecologist in writing rather than from a rule of thumb. Record it as a design constraint at the start, alongside the drainage and highway constraints, because it governs the layout.

Two habits make the target achievable. Keep the lit area away from the feature by setting road and parking layouts back from the boundary. And accept a dark buffer as a designed element of the scheme, shown on the layout drawing with a width, rather than as an absence of something.

Where a dark corridor cannot be maintained, say so early. Retrofitting a solution once the road adoption layout is fixed is rarely possible.

Luminaire selection, spectrum and shielding

Four specification decisions carry most of the outcome.

• Spectrum. Warm sources with low blue content are preferred, and a correlated colour temperature at or below 2700 K is the usual specification on ecologically sensitive schemes. Cooler white sources scatter further and are more disruptive to nocturnal species.
• Optics. Full cut off or flat glass luminaires with a zero upward light ratio, mounted level rather than tilted, keep the light on the surface it is meant to serve.
• Mounting height. Lower columns with tighter distributions light a smaller area than tall columns with wide optics, at the cost of more columns.
• Accessories. Shields, baffles, louvres and back light control cost almost nothing at design stage and are awkward to add later.

Obtrusive light limits in the lighting industry's own guidance, structured around environmental zones from dark rural to high district brightness, sit alongside the bat targets. They are not the same thing, and meeting a zone limit does not demonstrate that a hedgerow is dark enough.

Controls, timing and part-night regimes

A luminaire that is switched off is the most effective mitigation available, and controls are where schemes gain the most for the least cost.

Part night regimes that extinguish or deeply dim lighting through the core activity period address the hours that matter most. Presence detection on private accesses, car parks and service areas removes standing illumination entirely. Dimming profiles that reduce output after a set hour are the fallback where a highway authority will not accept switch off.

The obstacle is usually adoption. Highway authorities apply their own lighting standards to roads they will adopt, and those standards may not permit switch off. Raise it at the highway design stage, not at condition discharge, and consider private management of lighting on sensitive sections where the authority will agree to it.

Security lighting is the other recurring problem. A single unshielded floodlight on a PIR, installed by an occupier two years after completion, undoes a carefully modelled scheme. Where the risk is real, remove permitted development rights for external lighting by condition and say why in the committee report.

Modelling and how the results are presented

Present results at the feature, at the height bats fly, and in a form a reviewer can check.

The most common reporting failure is a plot at ground level only. A hedgerow four metres high can be comfortably below the target at the ground and well above it at two metres, and the two metre figure is the one that matters. Modelling light against a sensitive receptor rather than against a general standard is the same discipline that applies to glint and glare near an aerodrome.

Include the assumptions. Reflectance values, maintenance factor, and whether the model includes the proposed planting all change the answer.

Securing the scheme by condition

A condition requiring a lighting scheme to be submitted and approved is standard, and it is often drafted too loosely to be useful.

Ask for wording that names the outputs: the luminaire schedule, the isolux plots at stated heights including vertical illuminance at retained features, the control regime, and the target level agreed with the ecologist. Add a requirement that no external lighting other than that approved is installed, and tie the scheme to the ecological mitigation strategy so the two documents cannot drift apart.

A Lighting Assessment prepared alongside the Bat Survey gives the officer a coherent package, and it makes the condition straightforward to discharge.

Verification after installation

Commissioning measurements at the retained features, taken after dark with the scheme fully operating and the controls set, close the loop. Measure at the heights modelled, at points shown on a plan, and report against the target.

Discrepancies are normal. Luminaires get tilted during installation, a shield is omitted, a control node is not commissioned. Finding that in month one is a snagging item. Finding it in year three, after an ecologist records abandonment of a commuting route, is something else.

This is general guidance, not legal advice; check the position with your local planning authority and a licensed ecologist before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.