Choosing between 300mm and 400mm signal heads is a common decision point in traffic signal design. The wrong diameter costs money or visibility. This guide shows the 5 decision points that determine the right choice.
Why Diameter Matters
Signal diameter is one factor in visibility, alongside optical performance, aiming, background conditions and installation geometry. Drivers must be able to recognise the indication with adequate time to respond under the approved road design. Compare documented performance for 300mm and 400mm models; do not infer a guaranteed viewing distance from diameter alone.
Decision Point 1: Design Speed
Approach speed is one important input alongside sight distance, background and approved signal-layout requirements:
- Verify the approved approach-speed and visibility requirements before using a 60 km/h selection threshold.: Verify that 300mm meets the approved visibility and road-design requirements
- 60-80 km/h: Confirm whether a 300mm head with the proposed backplate meets the approved visibility requirements
- 80-100 km/h: Consider 400mm only where supported by the approved design and optical requirements
- For speeds above 100 km/h, obtain a site-specific visibility and signal-design review: select diameter through the approved optical and site assessment, not speed alone.
Confirm the required signal diameter with the road authority and the approved design. Product testing to EN12368 does not by itself assign a diameter to a road speed. Any claimed increase in driver decision time must be supported by the site geometry and visibility assessment, not inferred from diameter alone.
Decision Point 2: Road Class
Road classification affects diameter choice:
| Road Class | Diameter | Rationale |
|---|---|---|
| Local street | Select diameter from the approved layout and visibility assessment. | Low speed, low volume |
| Collector | Select diameter from the approved layout and visibility assessment. | Mixed use |
| Arterial | Select diameter from the approved layout and visibility assessment. | Higher speeds, longer sight distance |
| Highway | Select diameter from the approved layout and visibility assessment. | High-speed approaches: verify the required visibility |
| Freeway | Select diameter from the approved layout and visibility assessment. | Select diameter from the approved layout and visibility assessment. |
For urban, highway and rural approaches, confirm the diameter from the approved signal layout and visibility assessment; neither 300mm nor 400mm is automatically the safe choice.
Decision Point 3: Visibility Requirements
If a signal must be visible from a specified distance, verify the selected assembly’s optical evidence, installation geometry and applicable visibility requirements. Do not derive an approved installation from distance alone or an unverified size formula.
- Stopping sight distance depends on speed, perception–reaction time and braking conditions, including the applicable road gradient and friction assumptions.
- At 80 km/h, calculate stopping sight distance using the applicable design assumptions; speed alone does not establish a distance.
- Perception-reaction distance = speed (km/h) / 3.6 × time (s). For arithmetic illustration only, 80 km/h and 0.75 s give about 16.7 m. The 0.75 s input is not a recommended road-design reaction time; use the approved design assumptions.
- Determine the required visibility from the applicable road-design criteria and site geometry; 160m is not a universal minimum and must not be inferred from an unverified reaction-distance calculation.
Compare the documented visibility and optical performance of 300mm and 400mm models for the approved installation. Include viewing angle, background contrast, weather and glare in the assessment; diameter alone does not guarantee a particular viewing distance or a margin for every driver.
Decision Point 4: Sun Phantom Risk
Sun phantom is an apparent signal caused by external light when the light source is off. Compare the tested phantom performance of the complete optical unit, lens, visor and housing under the required conditions. A 400mm diameter alone does not establish a lower phantom risk than 300mm. Assess:
- Assess unwanted-light suppression through the documented optical design, visor and installation geometry.
- Deeper visor tunnel
- Verify contrast under the specified bright-background conditions
For signals facing low-angle sun, assess orientation, visor and backplate design, optical test evidence and site visibility before selecting the head diameter.
Decision Point 5: Cost and Infrastructure
Compare quotations for equivalent approved configurations:
| Diameter | Relative Cost | Mounting Hardware |
|---|---|---|
| 200mm | Obtain a quotation for the specified configuration. | Verify brackets and supports against actual equipment weight and wind loading. |
| 300mm | Obtain a quotation for the specified configuration. | Verify brackets and supports against actual equipment weight and wind loading. |
| 400mm | Obtain a quotation for the specified configuration. | Verify brackets and supports against actual equipment weight and wind loading. |
Larger signal heads can change equipment weight and wind loading, which may affect brackets, pole arms and foundations. Include the actual head, visor and backplate configuration in the structural review. Confirm any required mounting changes and the design scope with GAOQIAO before ordering to reduce the risk of field modifications.
Check Mixed-Diameter Configurations Against the Approved Layout
If different signal diameters are proposed on the same approach, have the responsible designer and authority review visibility, consistency and the applicable requirements. Document the approved arrangement and its rationale. Do not assume that mixing 300mm and 400mm heads is permitted merely because the choice is intentional.
Rule: Select signal diameters for each approach under the approved layout and authority requirements. Assess any mixed arrangement for visibility and indication consistency rather than using diameter alone to address sun phantom.
Conclusion
Choose between 300mm and 400mm only after reviewing the applicable road-authority requirements, approach speed, sight distance, background, sun exposure and mounting layout. Compare the tested optical performance and project quotations. Neither a universal speed threshold nor a fixed price premium establishes which head is suitable or safe for the site.
For 200mm, 300mm or 400mm signal-head enquiries, send the road class, design speed, required indication and approved project requirements. Ask GAOQIAO to confirm the available configuration, relevant test documents and quotation.
Get a Quote
Send us your road details and we will recommend the right diameter and provide a quotation.
Email: sky-sale@njgq.net
WhatsApp/Phone: +86-18805173545
Tel: +86-25-84471080
Frequently Asked Questions
Can I mix 300mm and 400mm on the same intersection?
Mixed diameters require review against the approved layout, visibility and authority requirements; intentional selection alone does not establish permission.
Does 400mm use more power than 300mm?
Compare the actual power demand of the selected models and operating modes. Use those values in solar sizing; do not infer a fixed percentage increase from diameter or LED count alone.
Which diameter is most common in urban areas?
Evaluate 300mm signals against the project’s visibility, optical and mounting requirements rather than an unsupported global market-share estimate.
What about the cost difference for the whole intersection?
For a four-leg intersection, compare quotations for the proposed 300mm and 400mm configurations using the same scope. Include any changes to pole and arm loading, structural design and installation cost. The cost difference must be established for the actual project rather than assumed to be 40-60%.
Related GAOQIAO Products
These public GAOQIAO catalogue pages are related to this article.
- Traffic signal controllersWired and solar controllers with 6-56 outputs.
- LED signal heads200mm, 300mm, 400mm; confirm model-specific EN12368 evidence.
- Signal polesVertical, signal arm and double-arm configurations.
- Spare partsLED modules, lenses, visors and accessories.
Related Resources
- Traffic Signal Controller Cabinet Guide
- Traffic Light Cost & Price Guide
- LED Traffic Light Buying Guide
- LED Traffic Light Installation Guide
- Contact GAOQIAO
Related guides: signal size and layout
Continue with these related resources for your traffic signal project.