Is Brighter Better?

Outdoor Parking Lot Lighting and Perceptions of Safety

By John D. Bullough, PhD, and David Pedler

The chastening reality underlying Americans' passion for the open road is that our vehicles spend 95 percent of their useful life parked at home or in one of the country's estimated 2 billion parking spaces.1 Most of us drive to workplaces that invariably adjoin parking lots, and many of our choices about where we go outside working hours are based on whether parking is conveniently accessible.2, 3 But while the availability of convenient parking lots may draw us to our destinations, considerations of safety and security are major factors in deciding whether we end up parking there.4

Aside from our feelings about a given parking lot, there are overriding objective considerations that go into how its lighting should be designed and specified. Does the lighting enable drivers to see each other and pedestrians at night? Can drivers and pedestrians readily identify parking lot surface irregularities and obstructions that might hinder movement through the space?

Multiple research studies have confirmed that the Illuminating Engineering Society's recommended minimum average horizontal illuminance of 2 lux5 is sufficient for the visual performance of drivers and pedestrians.6-8 But subjective considerations based on our perceptions also play a role, and there are strong indications that lighting considered technically sufficient for visual performance does not necessarily create the perception that a parking lot is actually safe.9

For example, can drivers and pedestrians — but especially vulnerable populations like elders and the physically disabled — feel confident about who or what else might be present in the space at night? Can the entirety of the space be confidently viewed as free from risks that might be concealed by shadows and other dark areas? This is where lighting design can play a key role.

Previous research has confirmed the conventional wisdom that a "brightly" or “well-lit” parking lot can be judged a safe and secure parking lot (Figure 1),9, 10 but more recent laboratory and field studies by LHRC researchers11, 12 and others have since demonstrated that illuminance is only one piece of the puzzle.

Figure 1. Higher average illuminance levels correlate with greater feelings of safety and security regardless of setting, whether in a large versus a small city (A) or in urban versus suburban (B) locations. The data from the two cities show stronger agreement that the lighting feels safe for average illuminance values >30 lx (A). Data from previous studies by Boyce et al.9 and Leslie.10

In 2020, LHRC researchers (formerly with the Lighting Research Center at Rensselaer Polytechnic Institute) performed laboratory and field studies examining illuminance levels, spectral distribution, and illuminance uniformity, which are factors that frequently have been investigated individually but rarely together. Starting in the laboratory, the researchers built a scaled physical model of a parking lot (Figure 2) to present combinations of four average illuminances ranging from 2.5 lx to 20 lx, three correlated color temperatures (CCTs) ranging from 2850 K to 5800 K, and three illuminance uniformity ratios ranging from 2:1 to 15:1 (maximum-to-minimum). For each of the conditions, participants whose views were positioned at a consistent height and distance from the scene were asked to rate perceived safety on a scale from -2 (very unsafe) to +2 (very safe).

Figure 2. Examples from the range of spectral distribution and illuminance uniformity scenarios that were presented to laboratory experiment participants.

The laboratory experiment showed that greater illuminance uniformity promoted greater perceptions of safety across all tested average illuminance levels (Figure 3A), and that only more modest gains in perceived safety could be realized by increasing the lighting's spectral distribution from 2850 K to 5800 K (Figure 3B). While the upward slope of the curves in both graphs in Figure 3 confirms that safety perception improves with higher average illuminance levels, the left panel (A) also shows there may be a limit to safety perceptions of low-uniformity lighting designs, even at relatively high (~20 lx) average illuminance levels.

Safety perception can also be promoted with greater efficiency by choosing lighting designs with greater uniformity, as shown in Figure 3A. Namely, in our laboratory experiment, the model parking lot receiving an average illuminance of 5 lx from a system with an illuminance uniformity of 2:1 received a similar safety rating to the one receiving 300% greater average illuminance (i.e., 20 lx) from a system with a lower uniformity of 15:1.

Figure 3. Safety ratings recorded for different illuminance uniformity (A) and spectral distribution (B) conditions for the four simulated parking lot lighting conditions depicted in Figure 2.

Building upon the results of the laboratory study, our research team next used these data to create a mathematical model for predicting perceptions of safety at the lighting design stage. (The resulting parking lot lighting safety tool, a downloadable Microsoft Excel workbook, is available in the original article.) The model was then tested in five physical parking lots with broadly varying lighting characteristics in the New York Capital Region to gauge predictions of perceived safety against actual feedback from study participants who visited the parking lots in random order at night (Figure 4). The statistical analysis revealed nearly perfect correlations between the model predictions and participants' responses to the statement: "If I were alone at night, I would feel safe in this parking lot” (r 2=0.982).

Figure 4. Lighting characteristics for the five sites in the New York Capital Region that were used to test the validity of the mathematical models described here. Perceived safety values indicated three lots felt "somewhat safe" (+1.0 to +1.9), one felt "neither safe nor unsafe" (0 to +0.9), and one felt "somewhat unsafe" (-1.0 to -1.9).

In answer to the question posed in the title of this article, brighter can be better but not in the way we might expect. Indeed, a parking lot with an average illuminance of 5 lx can appear safer to pedestrians than one with an average illuminance of 20 lx, especially if the lighting is more uniform.

Designers should be mindful of the broader factors that inform users' perceptions of parking lot safety at night. Our research shows that taking a more comprehensive approach to parking lot lighting design can create opportunities for substantial reductions in energy use (and light pollution) through the careful selection of luminaires, spectral distributions, and pole layouts that provide illuminance distributions that feel safe and secure. Less can be more!

This research was sponsored by the LHRC's Lighting Energy Partners (Northwest Energy Efficiency Alliance, BC Hydro, and Eversource).

References

1. Harrison D. America Has Too Much Parking. Really. Wall Street Journal. 2023.Available from: https://www.wsj.com/articles/parking-problem-toomuch-cities-e94dcecf

2. Inci E. A review of the economics of parking. Economics of Transportation. 2015;4(1):50-63. doi: 10.1016/j.ecotra.2014.11.001

3. Franco S. Parking Prices and Availability, Mode Choice and Urban Form. International Transport Forum Discussion Papers [Internet]. 2020. Available from: https://www.itf-oecd.org/sites/default/files/docs/parking-mode-choice-urban-form.pdf.

4. Ben Hassine S, Mraihi R, Lachiheb A, Kooli E. Modelling parking type choice behavior. International Journal of Transportation Science and Technology. 2022;11(3):653-64. doi: 10.1016/j.ijtst.2021.09.002

5. Illuminating Engineering Society. RP-20-14. Lighting for Parking Facilities. New York: Illuminating Engineering Society; 2016.

6. Fotios S, Cheal C. Obstacle detection: A pilot study investigating the effects of lamp type, illuminance and age. Lighting Research and Technology 2009;41(4):321-42. doi: 10.1177/1477153509102343

7.Bullough JD. Lighting Answers: Dynamic Outdoor Lighting Troy, NY: Lighting Research Center, Rennselaer Polyyechnic Institute; 2010. Available from: https://www.lrc.rpi.edu/programs/NLPIP/lightingAnswers/pdf/print/LADynamicOutdoor.pdf.Accessed 24 July 2024

8. Bhagavathula R, Gibbons RB. Light levels for parking facilities based on empirical evaluation of visual performance and user perceptions. LEUKOS 2020;16(2):115-36. doi: 10.1080/15502724.2018.1551724

9. Boyce PR, Eklund NH, Hamilton BJ, Bruno LD. Perceptions of safety at night in different lighting conditions. Lighting Research and Technology 2000;32(2):79-91. doi: 10.1177/096032710003200205

10. Leslie RP. A simple cost estimation technique for improving the appearance and security of outdoor lighting installations. Building and Environment 1998;33(2-3):79-95. doi: 10.1016/S0360-1323(97)00051-6

11. Rea MS, Radetsky LC, Bullough JD. Toward a model of outdoor lighting scene brightness. Lighting Research and Technology. 2011;43(1):7-30. doi: 10.1177/1477153510370

12. Rea MS, Bullough JD, Brons JA. Parking lot lighting based upon predictions of scene brightness and personal safety. Lighting Research and Technology. 2017;49(3):293-304. doi: 10.1177/1477153515603758.

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