Introduction

Typically, the spacing between streetlights on urban roads ranges from 25 to 50 meters. Generally speaking, the greater the street light distance, the better; however, this spacing is influenced by many factors. Excessively wide spacing between streetlights can lead to poor lighting performance. For example, uniformity may decrease, resulting in noticeable dark areas around streetlights located far from the ones on either side; or brightness and illuminance may be too low, causing the entire road to appear dim and failing to meet the visual requirements of drivers and pedestrians. Simply put, the primary factors determining street light distance are the nature of the road (e.g., industrial park roads, rural roads, urban roads) and the power rating of the streetlights. However, as professionals in the field, we should approach the issue from the perspective of road lighting design (ZGSM road lighting design), which involves road width, pole height, lighting requirements, and the optical characteristics of the luminaires, among other factors. This article will focus on analyzing how these factors influence street light distance, as well as the efforts ZGSM has made in this regard. Let’s dive into the main text to learn more.

Which factors have an impact on street light distance?

There are many factors that influence street light distance. The following sections will focus on pole height, light distribution, street light arrangement, and road lighting requirements. In fact, various parameters involved in lighting simulations—such as tilt angle, overhang length, and road surface characteristics—all affect street light distance. However, compared to the core factors mentioned above, the impact of these parameters is relatively minor, so they will not be discussed in detail here.

Pole height

Pole height is one of the key factors affecting street light spacing. Generally, the taller the pole, the greater the coverage area of the street light, and the spacing between streetlights can be correspondingly increased. The industry typically uses 3.5 to 5 times the pole height as a reference for planning street light spacing; for example, for a common 8-meter pole, the recommended spacing is approximately 25–40 meters. As spacing increases, both the cost of the poles and installation costs decrease. At the same time, this places higher demands on the luminaire’s light distribution (How to choose the right light distribution types for streetlights); not all luminaires can provide effective illumination at wider pole spacings. ZGSM compared the maximum pole installation spacing for the same light distribution at three heights—6 meters, 8 meters, and 10 meters—to help better understand this influencing factor. From the table, we can see that shorter poles have a limited illumination range, requiring closer street light distance to ensure road lighting meets design requirements. Specifically, for this lens and application scenario, a 6-meter pole is suitable for a 30-meter street light distance (we note that the Ti value does not meet requirements under these conditions, as the low pole height is a key factor contributing to a high Ti value). Taller poles, on the other hand, provide a wider illumination range, allowing for appropriately wider street light spacing while still meeting road design specifications. By comparing the data, we can see that, for this lens and application scenario, a 10-meter pole is suitable for a 50-meter street light spacing, while the 8-meter pole height suitable for a street light distance is 40 meters which is between the above two applications.

Light distribution

According to IESNA standards, we know that street light distribution is categorized into lateral light distribution and vertical light distribution. These factors determine the light’s projection capabilities; simply put, the former determines the light’s ability to illuminate the area in front of the fixture, while the latter determines its ability to illuminate the sides of the fixture. From this, we can see that vertical light distribution determines the effective range of street lights, and vertical light distribution is classified into short, medium, and long ranges. Below, we compare the most optimal pole spacing achievable for Type IIS, Type IIM, and Type IIIL at the same height (8 meters). Short vertical light distribution means the light emitted from the streetlight has a short throw, remaining close to the pole. As we can see, its suitable street light spacing is only 33 meters; although it can achieve a ground illuminance of 0.60 cd/m², this far exceeds the project’s M3 road lighting class requirements. Long vertical light distribution means the light emitted from the streetlight projects a long beam that extends far from the pole. As we can see, the optimal street light distance for this type is 43 meters, far exceeding that of Type IIS, and the illuminance also meets the 0.50 cd/m² requirement of the EN 13201 M3 lighting class (More about EN13201 road lighting standards). Type IIM falls between the two, while also offering good glare control. Click here to learn more about Glare and how to reduce it.

Street light arrangement

There are four main types of street light arrangements commonly used in road lighting: one-sided arrangement, two-sided arrangement, two-sided offset (staggered) arrangement, and median arrangement. Different street lighting arrangements are generally selected based on road width; for example, one-sided arrangements are suitable for narrow roads, symmetrical two-sided arrangements are often used on wide arterial roads, staggered arrangements are suitable for roads of medium width, and median arrangements are used on wide arterial roads with a median strip. Upon closer examination, it becomes clear that the luminaire arrangement in street lighting is also related to pole spacing (street light distance). When comparing a two-sided arrangement with a two-sided offset arrangement on roads of the same width, it is evident that the latter allows for greater spacing between streetlights. This difference is determined by the luminaire’s light distribution characteristics: the illuminance on the far lane is significantly lower than that on the lane adjacent to the luminaire; even within the same lane, the road surface illuminance is lower at points farther from the luminaire. Analysis of lighting simulation results for the same luminaire model under both arrangements shows that the spacing between streetlights in a two-sided offset arrangement (More Street luminaire arrangement)can reach up to 42 meters. This is because the offset arrangement allows light to be supplemented from both sides of the road to specific points on the pavement, thereby increasing brightness in low-light areas and ultimately enabling wider street light distance.

Road lighting class and road width

Generally speaking, major urban thoroughfares and expressways have high traffic speeds and heavy traffic volumes. To ensure traffic safety, lighting requirements tend to be higher, and the street light distance is set to be narrower. In contrast, residential side streets and rural roads have lower speeds and are primarily used by pedestrians, so lighting requirements are lower, and the street light distance can be appropriately increased. When the road layout remains unchanged, selecting different road lighting classes results in varying lighting standard requirements. The limit values for metrics such as road surface luminance, overall uniformity, longitudinal uniformity, and glare will change accordingly, ultimately affecting the street light space – rather than simply distinguishing based on the road types mentioned above. Below, we compare how street light spacing changes to meet the corresponding lighting class when the road lighting class changes. In this project, the road is a four-lane, two-way expressway with a 1-meter overhang and 10-meter-high poles. We compared how the same lens meets M2 and M3 road lighting classes under different pole spacing conditions. The table shows that to achieve the M2 lighting class, the street light distance (pole spacing) must be appropriately reduced to ensure uniformity (What’s light uniformity?) and brightness while minimizing glare. Conversely, to achieve the M3 lighting class, the street light spacing can be appropriately increased (from 40 meters to 50 meters). Road width also has a certain impact on street light spacing; if the lens is inappropriate or the road is too wide, the achievable street light spacing will often be reduced.

Why is street light distance important?

Impact on compliance with lighting standards

The spacing between streetlights is a key factor in determining whether various road lighting metrics meet standards, directly affecting average road surface brightness, overall uniformity, longitudinal uniformity, and glare control. Generally, the closer the street light distance, the higher the overall brightness of the road surface and the better the lighting uniformity, making it easier to meet the lighting specifications for the corresponding road classification.

Conversely, if the street light distance is too wide, the overlap area between adjacent luminaires will be significantly reduced. Sections of the road far from the light poles are prone to forming noticeable dark zones, resulting in road surface brightness and uniformity that fail to meet standards and rendering the lighting unable to meet the established lighting grade requirements. Even if the overall road surface brightness is forcibly increased by using higher-wattage luminaires, the excessive brightness of the luminaire’s light-emitting surface will cause severe glare, impairing drivers’ visual perception and directly compromising road safety.

Impact on project cost and long-term O&M costs

Given a fixed total road length, the greater the street light distance, the fewer light poles, luminaires, cables, and foundation embedments required for the project, which effectively reduces the overall costs of equipment procurement and on-site installation. If the spacing between lights is too dense, equipment and construction costs will increase significantly, resulting in unnecessary waste of resources.

At the same time, a well-designed luminaire light distribution pattern allows for wider street light distance while meeting all lighting standards with lower luminaire power ratings. This optimization approach reduces both the number of luminaires and the power consumption per unit, effectively lowering initial construction costs and significantly reducing long-term operational expenses related to electricity consumption, street light maintenance (About street light maintenance), and replacement. For suppliers and design firms, such an optimized solution that balances compliance and cost-effectiveness offers a stronger competitive edge, making it easier to gain an advantage in competitive bidding for similar projects.

Impact on ensuring the safety at night

The distance between streetlights directly affects the safety of the roads at night and is critical to the visual experience of motorists, pedestrians, and non-motorized vehicles users. The spacing will be such that the light will flow smoothly over the surface of the road, without the viewer seeing distinct light and dark discontinuities or localized dark areas. Proper and even road lighting enables drivers to clearly see lane markings, obstacles, and traffic conditions when passing intersections. This eliminates visual blind spots, allows drivers to anticipate changing road conditions in time, and helps prevent accidents (such as lane departures, rear-end collisions, or mistakes during emergency maneuvers) caused by poor visibility or delayed reaction.

If the streetlights (ZGSM street lights) are too far apart, the road illumination is inconsistent and intermittent with sharp transitions between light and shadow. Then the eyes of drivers are forced to continuously adapt to changing light intensities, which can easily cause visual fatigue and perceptual errors, and thus greatly increase driving hazards. Moreover, the uniform and steady lighting at night allows pedestrians and non-motor vehicle users to clearly see the immediate environment and road conditions, which can effectively help them to avoid various hazards such as minor collisions or sudden emergencies, thus comprehensively improving the overall safety and stability of road travel at night.

ZGSM street lighting solutions

Summary

Determining the optimal spacing between streetlight poles is a critical aspect of road lighting design, directly affecting lighting quality, project costs, and nighttime traffic safety, among other factors. This article first discusses the fundamental role of pole height in determining street light distance: the taller the pole, the wider the illumination range, allowing for greater spacing between poles; however, this also places higher demands on the light distribution (What’s IESNA light distribution of street lights?). Second, the vertical light distribution (short, medium, or long) within the light distribution pattern determines the longitudinal projection capability of the light; a long light distribution allows for greater pole spacing (street light distance) at the same height. Third, the streetlight layout (single-sided, two-sided, two-sided offset (staggered), or median arrangement) is not only related to road width but also, for the same width, allows for greater pole spacing in a staggered two-sided layout due to complementary lighting from both sides. Furthermore, the road lighting class and road width also influence pole spacing: higher classes (e.g., M2) have stricter requirements, necessitating a reduction in street light distance; for lower classes (e.g., M3), street light distance can be appropriately increased. This article also presents a comparison of actual data from ZGSM under different pole heights, light distribution patterns, and lighting classes, demonstrating how to optimize street light spacing while meeting regulatory requirements. In summary, determining the optimal street light distance requires a comprehensive consideration of multiple factors, including pole height (What factors decide height of street lights?), light distribution, layout, and lighting class; ZGSM’s LED lighting solutions provide a viable path for optimization.

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