Links to other memoranda in this series:

1. ComparativeBenefits of Trams and Buses in Urban Public Transport
2. Dispelling the myth that trams need special seperate segregated path through traffic
3. “Glue-in-the-Road” TramTrack Systems versus Conventional Deep Excavation Methods
4. Why trams are essential to improve city productivity
5. Overhead wire-freeTechnologies for Tram Propulsion
6. Birmingham Tramworks Show Utilities Don’t Always Need to Be Moved: Minimal-Dig Track Methods in Action – A UK & International Perspective

Draft 5

Technical Memorandum 2: Green Wave Traffic Light Pre-emption (GWTP) in Modern Tram Systems

Subject: Removing the Myth That Trams Require Special Segregated Paths Through Traffic
Prepared for: Policy Makers and Infrastructure Planners
Prepared by: Bristol and Bath Tram Association / West of England Transport Association
Date: June 2025

1. Executive Summary

A common misconception in UK transport planning is that trams must operate entirely off-street or with full segregation to ensure effective performance. This assumption is incorrect. Modern tram systems utilise Green Wave Traffic Light Pre-emption (GWTP) to operate efficiently in mixed traffic, allowing trams to glide through congested areas with minimal delay. GWTP enables corridor-wide signal coordination well beyond the basic “signal priority” used for buses [1][2].

2. Green Wave vs. Signal Priority

In the UK, traffic signal priority (SP) for buses is widespread but typically limited to activating lights on a per-intersection basis. This has only marginal impact since buses remain trapped by congestion and lack corridor-wide prioritisation [3].

GWTP is fundamentally different:

• It coordinates all signals along a corridor in advance of an approaching tram to form a continuous “green wave.”
• Side streets are held on red to prevent blocking the tram’s path.
• Trams appear to “sail through” traffic — enhancing punctuality and reducing interference with general traffic.
• Crucially, this approach requires no dedicated tram lanes, freeing up space for other road users.

Unlike SP, GWTP offers a high degree of reliability and throughput for trams, with minimal need for physical segregation [4][5].

Case studies from cities including Strasbourg, Zurich, Melbourne, and Manchester demonstrate that high-performing tram systems frequently operate in shared road space, benefiting from adaptive traffic signal systems that create a smooth, uninterrupted route [2][3][4]. In many cases, general traffic flow also improves under GWTP regimes [6]. Memo 3 further supports this by showing that modern embedded track systems enable shared-space installations without extensive utility diversions [12].

3. Why Buses Do Not Use GWTP

GWTP is not viable for buses because achieving the same capacity as a tram network requires 4 to 5 times more buses. These buses then begin to interfere with each other at signals, making it impractical to coordinate a green wave. Trams, by contrast, concentrate passengers into a single long vehicle, which avoids this interference. Trams follow fixed routes with consistent acceleration and braking, enabling predictable signal coordination [6].

A detailed video explanation is available via Bath Trams [10]. The World Bank also acknowledges this challenge, supporting the case for fixed-rail systems where high throughput and reliability are essential [11].

4. Why the Myth Persists

The perception that trams must be fully segregated stems from:

• Legacy systems without signal coordination.
• Misunderstanding of modern traffic management technologies.
• Transport appraisal frameworks biased towards conventional rail.
• Confusion between fully segregated “light rail” and modern city trams.

This confusion leads to inflated project cost estimates in the UK compared to more cost-effective, on-street European models [1].

5. What Mixed-Traffic Operation Means

Operating in mixed traffic does not mean exposing trams to random, unpredictable delay. When paired with GWTP and embedded track technologies (see Technical Memo 3), trams in shared roadways can achieve:

• On-time performance >90%
• Shorter, more reliable journey times
• Seamless interaction with other road users

Because trams run on fixed rails and follow precise acceleration profiles, signal systems can anticipate their movement and coordinate light cycles accordingly. This predictable behaviour is what makes pre-emption possible [5][6].

6. What is GWTP and How Does It Work?

6.1 How It Works

• Detection and Communication: Tram sends signal via GPS or radio to upcoming traffic controllers.
• Signal Pre-emption: Not just the next junction but multiple lights are coordinated in advance.
• Green Wave Creation: Lights turn green in sequence to match tram’s expected pace.
• Real-Time Adaptation: The system can dynamically adjust based on vehicle conditions [6].

6.2 Key Features

• Pre-emption vs. Priority: Pre-emption guarantees a green path; priority merely shortens wait time.
• Side Street Control: Minor roads are briefly held to prevent obstruction.
• Integration with Adaptive Systems: Compatible with SCOOT, SCATS, and AI-based systems [7][8].
• Safety: All systems remain compliant with road safety regulations.

7. Benefits to Tram Operations and General Traffic

• Improved Journey Times: Near-continuous movement through junctions.
• Service Reliability: Predictable schedules and consistent headways.
• Lower Capital Costs: Eliminates need for full segregation.
• Urban Integration: Maintains access and vibrancy of streets.
• Motorist Benefits: Smoothed traffic light cycles benefit all users [1][4].

8. Global Case Studies

(See Memo 1 for general tram benefits; Memo 3 for track technology and construction disruption mitigation.)

9. Conclusion

GWTP enables modern tram systems to operate efficiently, safely, and economically in mixed-traffic conditions—dispelling the outdated notion that trams require full segregation. Signal pre-emption creates a green corridor that allows trams to move through cities as though congestion were not present. This technology underpins the economic, operational, and political viability of surface-running tramways in modern urban contexts.

10. References

1. UITP (2018). Tram Best Practices – Urban Tram Operations.
2. Swiss Transport Research Conference (2019). Tram Signal Priority in Zurich.
3. Transport for Greater Manchester (2020). Metrolink Signal Integration.
4. VicRoads (2017). Melbourne Tram Signal Priority Systems.
5. Gartner, N.H. et al. (2001). Traffic Flow Theory: State of the Art. TRB.
6. Transportation Research Board (2010). Transit Signal Priority Manual.
7. TriMet (2006). Transit Signal Priority Evaluation. Portland. https://trimet.org/pdfs/publications/transit-signal-priority.pdf
8. Wikipedia (2024). Sydney Coordinated Adaptive Traffic System. https://en.wikipedia.org/wiki/Sydney_Coordinated_Adaptive_Traffic_System
9. Wikipedia (2024). King Street Transit Priority Corridor. https://en.wikipedia.org/wiki/King_Street_Transit_Priority_Corridor
10. Bath Trams (2024). Why You Get Bus Jam if You Try to Run a Large City on Buses Alone. https://bathtrams.uk/why-you-get-bus-james-if-you-try-to-run-a-large-city-on-buses-alone-not-trams
11. World Bank (2020). Improving Urban Transport Performance through Strategic Modal Priority.
12. Technical Memorandum 3 (2025). Embedded Track Systems vs. Deep Excavation Methods.

Links to other memoranda in this series:

1. ComparativeBenefits of Trams and Buses in Urban Public Transport
2. Dispelling the myth that trams need special seperate segregated path through traffic
3. “Glue-in-the-Road” TramTrack Systems versus Conventional Deep Excavation Methods
4. Why trams are essential to improve city productivity
5. Overhead wire-freeTechnologies for Tram Propulsion
6. Birmingham Tramworks Show Utilities Don’t Always Need to Be Moved: Minimal-Dig Track Methods in Action – A UK & International Perspective