Street Lighting

What Transportation Engineers Mean by “Street Lighting”

Street lighting is the engineered provision of roadway, intersection, path, and public-space illumination to improve night-time visibility, safety, security, and comfort. In transportation engineering, lighting design balances five priorities: safety outcomes (collision reduction and personal security), visual performance (seeing distance, contrast, and glare control), environmental stewardship (dark-sky compliance and wildlife sensitivity), lifecycle cost (energy and maintenance), and community character (color rendition and aesthetics).

This guide distills the essentials practitioners, students, and decision-makers search for: which standards to follow, how to size mounting heights and spacings, what photometric metrics matter (illuminance, luminance, veiling luminance), how to limit glare and skyglow, when to choose LED vs. HPS, what CCT/CRI do for visual tasks, how to deploy smart controls, and how to manage cost, procurement, and asset inventories. Along the way, you’ll find practical equations, checklists, and field tips to help your designs pass plan review and perform on the street.

Did you know?

Well-designed LED conversions can reduce energy use by 50–70% while improving uniformity and color rendering—if optics and CCT are chosen for the context.

Design for visibility first, then tune energy, controls, and environmental performance.

Design Standards & Target Levels

Street lighting criteria are typically based on roadway classification (freeway, arterial, collector, local), area type (urban, suburban, rural), pedestrian conflict level, and crime/safety objectives. Designers select targets for average horizontal illuminance on the roadway, luminance (in some methodologies), uniformity ratios, and threshold increment (disability glare). For crosswalks, intersections, transit stops, and shared-use paths, pedestrian-centric illuminance and vertical illuminance become critical for face recognition and yielding behavior.

Inputs: Roadway class, number of lanes, median/shoulder width, design speed, pedestrian activity rating, intersection type, environment (coastal, desert, snowy), and adjacent land uses.
Outputs: Target E_avg (lux), E_min/E_avg uniformity, veiling luminance ratio, and vertical illuminance at crosswalks (often 5–20 lux depending on risk).
Special zones: School areas, trail crossings, roundabouts, tunnels/underpasses, and mid-block crosswalks often warrant higher vertical illumination and stricter glare control.

Design Tip

Start from the most vulnerable user: size lighting for pedestrians at crossings first, then extend to roadway lanes and approaches.

Luminaires, LEDs, Optics & Color Quality

Modern street lighting is dominated by LED luminaires with precise optics that shape light distribution (Types I–V). Key decisions include drive current (efficacy vs. longevity), correlated color temperature (CCT), color rendering index (CRI or TM-30 fidelity/gamut), uplight rating (BUG system), and housing durability (IP rating, corrosion resistance). The right optic improves uniformity and reduces wasted light, while the right CCT balances visual acuity, comfort, ecology, and policy.

Distribution type: Select I–III for roadways; IV for wider medians/parking perimeters; V for open areas. Pair with roadway geometry and mounting height.
CCT & CRI: 2700–3000 K often preferred in residential/dark-sky areas; 3000–4000 K for arterials/centers. Higher CRI/TM-30 fidelity improves object recognition.
Thermal design: Lower drive currents reduce lumen depreciation and extend life; verify L₇₀/L₉₀ at project temperatures.
Shielding: House-side shields and zero-uplight optics minimize spill-light into homes and the night sky.

Photometric Fundamentals

Photometry translates luminaire output into the visibility people experience. Three concepts dominate roadway design: illuminance on the pavement (lux), luminance perceived by drivers (cd/m²), and vertical illuminance at pedestrian height for facial recognition and yielding. Designers evaluate these using IES files in lighting software, then confirm with on-site measurements.

Point Illuminance (Idealized)

\( E = \dfrac{I(\theta)}{r^2} \cos \theta \)

\(E\)Illuminance at point (lux)

\(I(\theta)\)Luminous intensity at angle \(\theta\) (cd)

\(r\)Distance from luminaire (m)

\(\cos \theta\)Incidence correction

Vertical Illuminance at Crossings

\( E_v \approx \sum \dfrac{I(\theta,\phi)}{r^2} \cos \alpha \)

\(E_v\)Vertical illuminance (lux) at 1.5–1.8 m

\(\alpha\)Angle to vertical target

While illuminance is easier to compute and verify, luminance-based methods can better reflect driver experience on asphalt with varying reflectance. For pedestrian safety and CCTV performance, ensure adequate vertical illuminance facing drivers and from curb ramps into the crosswalk.

Uniformity, Disability Glare & Visual Comfort

Uniformity affects how reliably drivers can detect objects across the pavement. Two common metrics are E_avg/E_min (or its inverse) and E_max/E_min. Poor uniformity produces bright/dark patches that conceal hazards. Disability glare (veiling luminance) reduces contrast; discomfort glare annoys and can prompt complaints—both must be controlled through optic selection, mounting height, cut-off shields, and CCT.

Uniformity Ratios (Concept)

\( U_1 = \dfrac{E_{\text{avg}}}{E_{\text{min}}}, \quad U_2 = \dfrac{E_{\text{max}}}{E_{\text{min}}} \)

\(E_{\text{avg}},E_{\text{min}},E_{\text{max}}\)Average, minimum, maximum pavement illuminance

Important

High E_avg with poor uniformity is not “good lighting.” Optimize distributions and spacings first; add lumens only if targets are still unmet.

Poles, Foundations, Spacing & Layout Patterns

Pole selection ties photometry to structural and roadside safety constraints. Designers balance mounting height, outreach (arm length), setback from curb, foundation capacity (wind load), and placement relative to driveways and utilities. Layout patterns—one-sided, staggered, opposite, median—shape uniformity and cost.

Mounting height: Taller poles improve uniformity and spacing but may increase glare if optics are not full cutoff.
Setback & frangibility: Use breakaway bases in high-speed zones; verify clear zone requirements.
Pattern choice: Staggered layouts often deliver better uniformity for two-lane roads; opposite or median mounting suits multi-lane arterials.

Rule-of-Thumb Spacing (Starting Point)

\( S_0 \approx k \times H \)

\(S_0\)Initial spacing estimate (m)

\(H\)Mounting height (m)

\(k\)Factor 3–6 (refine with photometry)

Use the rule-of-thumb only to seed software. Final spacing must be refined against target illuminance, uniformity, and vertical illumination, especially near crossings and bus stops.

Energy, Smart Controls & Communications

LEDs dramatically reduce energy use; controls add another 20–40% savings while enabling asset visibility. Core strategies include astronomical scheduling, adaptive dimming (by time-of-night or traffic), motion-based brightening for trails/alleys, and remote fault detection. Communications options include ANSI C136.41 (twist-lock) photocell nodes, cellular/NB-IoT mesh, or powerline carrier—each with trade-offs in latency, cost, and resilience.

Dimming profiles: 100% at peak, 70–80% at shoulder, 40–60% late-night where policy allows. Always preserve crosswalk vertical illumination.
Metering & billing: Networked nodes enable per-fixture energy tracking and faster outage response.
Cybersecurity: Require encrypted communications, role-based access, and periodic firmware updates.

Safety & Human Factors

Lighting supports task visibility (detecting pedestrians, obstacles, and edge lines), but also perceptions of safety. For motor vehicle safety, ensure consistent luminance on approach, enhanced vertical illuminance at decision points, and minimized disability glare. For vulnerable users, emphasize uniform, shadow-free crossings, well-lit waiting areas, and good color rendition for facial recognition.

Crosswalks: Provide upstream aimed luminaires for driver-facing vertical light; consider bollard-height supplemental lights for ramps.
Roundabouts: Light entries/exits and central island to reveal geometry; avoid over-lighting that masks retroreflective guidance.
Transit stops: Vertical light at curb edge, seating, and information signs; coordinate with CCTV and shelter design.

Light Pollution, Dark-Sky & Ecology

Light at night can scatter into the sky (skyglow), intrude into homes (light trespass), and disturb wildlife. Good street lighting delivers visibility without polluting the night environment. Choose zero-uplight optics (U0), low backlight near residential (B0–B1), and limit high-angle forward light (G ratings). Select warmer CCTs and apply dimming in low-activity periods where policy and ecology permit.

Consideration

In coastal and habitat corridors, use shielded, warm (≤3000 K) luminaires and reduce output during migration periods when feasible.

Maintenance, Inventory & Risk Management

Lighting is an asset class. A proactive program tracks fixtures, poles, wiring, and controls; schedules cleaning, aiming checks, and electrical inspections; and responds quickly to outages. With LEDs, failures are less frequent but more variable—drivers, surge protection devices (SPD), and photocontrols often fail before LEDs. Keep spare nodes and SPDs and test residual life during night rides.

Night audits: Quarterly drive-throughs or sensor-driven fault alerts verify performance and catch tilt/aim issues.
Vegetation: Branches can block photometry; coordinate pruning cycles with illumination priorities.
Electrical safety: Inspect grounding, corrosion, and door security; document any knockdowns and replace with like-for-like photometrics.

Field Note

Most “dark spots” after LED swaps trace to wrong optic types or tilted arms. A few degrees of aim correction can restore uniformity without adding fixtures.

Cost, Procurement & Funding

Evaluate projects on total cost of ownership: capital (fixtures, poles, wiring, nodes), energy, maintenance labor/parts, and risk contingencies. For conversions, map existing inventory and prioritize corridors with poor uniformity, high energy use, or safety concerns. Procurement can be design-bid-build, performance contracts, or utility-owned tariffs; each path changes risk, pricing, and control over standards.

Spec clarity: Require TM-21 lifetime data, driver replaceability, SPD ratings, BUG ratings, dimming capability, and photometric submittals (.ies files) matching catalog numbers.
Commissioning: Verify node registrations, dimming profiles, time sync, and photocell orientation; capture as-builts in the asset system.
Funding: Blend energy incentives, safety grants, and corridor rebuild budgets; include a retiming/controls optimization phase post-install.

Performance KPIs & Case Insights

Define success up front and measure it. Typical key performance indicators include average and minimum illuminance vs. target, uniformity ratio, vertical illuminance at crossings, outage rate, repair time, energy intensity (kWh per lane-km), and resident complaint trends. Record before/after night-time crash rates and near-miss observations at high-risk crosswalks.

Visibility: E_avg, E_min, vertical E at 1.5 m, and veiling luminance indicators.
Reliability: Outage percentage, mean time to repair, node connectivity rate.
Efficiency: kWh savings vs. baseline, peak demand reduction, adherence to dimming schedules.

Simplified Energy Savings

\( \%\text{Savings} \approx 1 – \dfrac{P_{\text{LED}} \times H_{\text{dim}}}{P_{\text{HPS}} \times H_{\text{base}}} \)

\(P_{\text{LED}}, P_{\text{HPS}}\)Fixture power (W)

\(H_{\text{dim}}, H_{\text{base}}\)Annual operating hours (with/without dimming)

Street Lighting: Frequently Asked Questions

What CCT should I specify?

Context matters. 2700–3000 K for residential/dark-sky areas reduces blue content and can improve comfort; 3000–4000 K on arterials balances acuity and color rendering. Always pair with good optics and glare control.

How do I choose mounting height?

Start with roadway width and desired uniformity. Taller heights (9–12 m) increase spacing and uniformity on wide roads; 6–8 m often fit locals. Validate in software and field-aim to control glare.

Do smart controls really save more?

Yes—dimming profiles, adaptive schedules, and rapid fault detection typically add 20–40% savings beyond LED efficacy, while improving uptime and safety.

How can I reduce complaints about glare or spill-light?

Use full-cutoff optics, warmer CCTs in sensitive areas, house-side shields, correct arm tilt (0° unless otherwise specified), and confirm pole locations relative to windows.

Is higher illuminance always better?

No. Excess light can increase glare and skyglow without improving visibility. Meet targets with good uniformity; don’t chase high averages.

Conclusion

Street lighting is more than swapping fixtures: it’s a systems design exercise that blends photometry, human factors, environment, and lifecycle economics. Start with clear targets based on roadway class and pedestrian risk. Choose optics and mounting that deliver uniformity with minimal glare, specify CCT and CRI appropriate to context, and protect the night sky with zero-uplight distributions and shields.

Combine efficient LEDs with smart controls and a disciplined maintenance program to keep performance high for years. Measure what matters—visibility, reliability, and community satisfaction—and iterate. When lighting feels “invisible,” users simply see better, feel safer, and your network saves energy night after night.

Design for people, respect the night, and manage assets like the critical infrastructure they are.