300mm vs 400mm Traffic Signal Head: 5 Decision Points
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 head diameter directly affects visibility distance. A driver needs to identify the signal color from a distance that allows safe stopping at the design speed. The 300mm head is visible from approximately 200m, while the 400mm extends visibility to 300m or more.
Decision Point 1: Design Speed
The most important factor is the approach speed of the road:
- Up to 60 km/h: 300mm is sufficient
- 60-80 km/h: 300mm with backplate is acceptable
- 80-100 km/h: 400mm strongly recommended
- Above 100 km/h: 400mm required
GAOQIAO matches diameter to the road's design speed per MUTCD and EN12368 standards. The 400mm head provides an additional 1-2 seconds of decision time at high speeds.
Decision Point 2: Road Class
Road classification affects diameter choice:
| Road Class | Diameter | Rationale |
|---|---|---|
| Local street | 200-300mm | Low speed, low volume |
| Collector | 300mm | Mixed use |
| Arterial | 300-400mm | Higher speeds, longer sight distance |
| Highway | 400mm | High speed, maximum visibility |
| Freeway | 400mm | Mandatory |
For most urban roads, 300mm is the default. For highways and high-speed rural roads, 400mm is the safe choice.
Decision Point 3: Visibility Requirements
If the signal must be visible from a specific distance, calculate backward:
- Stopping sight distance formula uses design speed and perception-reaction time
- At 80 km/h, stopping distance is approximately 100m
- Plus perception-reaction: 80 × 0.75 = 60m
- Total minimum visibility: 160m
300mm head provides about 200m visibility under good conditions. 400mm extends this to 300m, giving a comfortable margin for adverse weather, sun glare, or older drivers.
Decision Point 4: Sun Phantom Risk
Sun phantom is when direct sunlight makes an unlit signal appear lit. The 400mm head with visor has a much lower sun phantom risk than 300mm because:
- Larger lens area allows more backlight blocking
- Deeper visor tunnel
- Better contrast ratio in bright conditions
GAOQIAO recommends 400mm for any signal that faces east or west (sun in driver's eyes) regardless of speed.
Decision Point 5: Cost and Infrastructure
Cost difference is significant:
| Diameter | Relative Cost | Mounting Hardware |
|---|---|---|
| 200mm | 1.0x (baseline) | Lightweight |
| 300mm | 1.4-1.6x | Standard |
| 400mm | 2.0-2.5x | Heavy-duty required |
Larger signal heads require heavier mounting hardware, stronger pole arms, and sometimes upgraded foundations. GAOQIAO includes these in the system design to avoid costly field modifications.
Common Mistake: Mixing Diameters
Mixing 300mm and 400mm on the same approach is allowed if intentional, but should be documented. GAOQIAO has seen installations where 400mm was used for one signal and 300mm for others without clear rationale, creating driver confusion.
Rule: Use one diameter per approach unless there is a specific reason (like sun phantom mitigation for one direction).
Conclusion
Choose 300mm for most urban roads at 60-80 km/h. Upgrade to 400mm when design speed exceeds 80 km/h, when facing east-west sun, or when the road classification requires it. The 1.5-2x cost premium is justified by safety and visibility.
GAOQIAO supplies 200mm, 300mm, and 400mm signal heads in all standard configurations. Send us your road class and design speed and we will return a sized 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?
Yes, if intentional. Document the reason. Mixing without reason creates driver confusion.
Does 400mm use more power than 300mm?
Yes, about 30-40% more due to more LEDs. Factor this into solar sizing if applicable.
Which diameter is most common in urban areas?
300mm accounts for approximately 80% of urban intersections worldwide.
What about the cost difference for the whole intersection?
For a 4-leg intersection, 400mm throughout costs about 40-60% more than 300mm. The pole and arm costs also increase.
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 with EN12368 certification.
- Signal polesVertical, signal arm and double-arm configurations.
- Spare partsLED modules, lenses, visors and accessories.