Lighting a car park: illuminance, uniformity and curfews

Car park lighting fails at planning for one of two reasons: the scheme lights the tarmac well and spills far too much light onto the houses behind, or it satisfies the neighbours and leaves the parking area so patchy that the police architectural liaison officer objects. The design has to hold both ends at once, and the evidence that shows it does is an isolux drawing with calculated vertical illuminance at the nearest residential windows.

Two reference points frame the argument. BS EN 12464-2 gives maintained illuminance targets for outdoor work areas including car parks, graded by traffic intensity. The Institution of Lighting Professionals guidance note on the reduction of obtrusive light gives limits on light trespass, luminaire intensity and upward light by environmental zone, before and after curfew. A Lighting Assessment that cites one and ignores the other will be sent back.

Balancing safety standards against obtrusive light

The obtrusive light limits are set by environmental zone, and choosing the zone is the first decision in the report. Zones run from E0, intrinsically dark areas such as a National Park, through E1 for rural surroundings, E2 for low district brightness such as village and suburban edges, E3 for medium district brightness in suburban areas, to E4 for high district brightness in town centres and industrial areas.

Zone choice changes the permitted light trespass at a window by an order of magnitude between E1 and E4, so do not assert it. Justify it from the surroundings: the character of the area, the existing street lighting, whether the site sits within a settlement boundary, and any dark-sky or ecological designation nearby. An applicant who claims E4 for a retail car park backing onto open countryside will lose that argument in the consultation response.

The counterweight is the safety case. Where the car park serves a use operating after dark, there is a legitimate expectation of enough light for surveillance and for safe movement between vehicle and entrance. Say which standard you are designing to and which traffic category you have selected, rather than quoting a lux figure without its source.

Horizontal and vertical illuminance targets

Horizontal illuminance on the parking surface is what people think of as car park lighting. Vertical illuminance is what actually matters for two separate purposes, and they pull in opposite directions.

Inside the site, vertical illuminance at around 1.5 m above ground determines whether a person can recognise a face at distance and whether a camera can produce a usable image. Outside the site, vertical illuminance in the plane of a neighbour's window is the measure of light trespass that the obtrusive light guidance limits.

Report both. Give the calculated maintained horizontal illuminance across the parking area, the vertical illuminance at the assessment height within the site, and a table of vertical illuminance at each nearby residential window with the pre-curfew and post-curfew limits alongside. Name the window: "first floor rear bedroom, 14 the road behind" tells an environmental health officer where to stand.

Uniformity and why it matters more than average level

Overall uniformity is the ratio of the minimum illuminance to the average across the assessed area. A car park lit to a high average with poor uniformity is worse in practice than one lit to a lower average evenly, because the eye adapts to the bright patches and the dark ones read as darker still.

Poor uniformity is usually a spacing problem. Columns too far apart for their height produce scalloped pools with dark gaps between. The fix is either closer spacing or a wider distribution optic, and the second is generally better for neighbours, because it lets you hold the same uniformity from lower columns with less light thrown sideways.

State the calculated uniformity for each assessed area separately: the parking bays, the circulation aisles, the pedestrian route to the entrance, and the service yard if there is one. A single site-wide figure hides the dark corner.

Column height, spacing and spill at the boundary

Lower columns look less intrusive on a drawing and often perform worse at the boundary, because holding the same illuminance from a lower mounting height needs either more columns or luminaires aimed more steeply, and steeper aiming throws light further horizontally.

Four moves reduce boundary spill without wrecking the lighting design.

• Choose an asymmetric optic that puts the beam onto the parking surface with zero upward light and a sharp back cut-off.
• Set columns inboard of the boundary so the last row throws light back into the site rather than across it.
• Use rear shields or house-side shields on the boundary row, and show them in the luminaire schedule rather than mentioning them in the text.
• Set the tilt to zero. Tilted flat-glass luminaires are the single most common cause of complaints after a scheme is built.

Where the boundary adjoins a hedgerow, treeline or watercourse used by bats, light levels matter more than amenity alone. The joint guidance from the Institution of Lighting Professionals and the Bat Conservation Trust expects dark corridors to be maintained, and a lighting scheme that ignores it can undo an ecological mitigation strategy agreed elsewhere in the application.

Curfew regimes and part-night operation

A curfew is the time after which tighter obtrusive light limits apply. It is commonly set at 23:00 and often defined by condition to run until 07:00, though the hours are a matter for the authority.

Part-night operation is the usual way to meet post-curfew limits. Options run from full switch-off, through dimming to a percentage of output, to reducing to a security-only subset of luminaires covering the entrance and the pedestrian route. Full switch-off is simplest to condition and is sometimes unacceptable where the car park serves shift workers or a late-opening use.

Set out the operating regime explicitly, hour by hour, and show the calculation for the post-curfew scenario as well as the full-output one. An assessment that models only the fully lit condition has not tested the case the neighbours care about. The same logic applies to operational timing more widely, as night deliveries and servicing noise at commercial sites shows for the acoustic side.

Isolux drawings a reviewer can check

The drawing is the evidence. Make it auditable.

An unstated maintenance factor is the detail that most often undermines a lighting report. A scheme calculated at initial output looks compliant on day one and exceeds the trespass limits never, because the numbers were never the maintained ones. State the factor, and state the cleaning and replacement assumption behind it.

Conditions councils impose on car park lighting

Expect some combination of the following, and design for them rather than arguing after the event. A condition requiring the lighting scheme, including luminaire specification and isolux plots, to be approved before installation. A curfew condition fixing the hours and the post-curfew regime. A limit on vertical illuminance at named windows. A requirement for zero upward light. And a prohibition on additional external lighting without further consent.

Where a scheme is already built and generating complaints, the planning condition is not the only control. Artificial light from premises can be a statutory nuisance under the Environmental Protection Act 1990, subject to exemptions for certain premises such as airports and goods vehicle operating centres. That route runs separately from planning enforcement, as statutory nuisance after planning permission is granted explains.

Get the isolux plots, the curfew regime and the window-by-window table into the first submission. Lighting objections that arrive at week six are the ones that push a minor application past thirteen weeks.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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