Isolation from Mixed Traffic Is Essential to Unlock the Full Potential of Autonomous Vehicles
Operating autonomous vehicles in mixed traffic with human-driven vehicles imposes two fundamental limitations. First, sharing roads with conventional vehicles causes delays through traffic congestion. Second, unpredictable behaviour of human drivers and pedestrians, together with current traffic regulations, prevents autonomous vehicles from realising the full benefits of automated vehicle-to-vehicle coordination – optimal distance control, prioritised merging, dynamic road load management, and automatic compliance with radio-transmitted vehicle-to-infrastructure (V2I) commands.
To overcome these limitations, physically segregated road sections or physically segregated lanes must be introduced, reserved exclusively for highly automated vehicles operating in fully automated mode without a driver on board. These segregated sections and lanes require special traffic management rules, including dedicated road signs, road markings, entry and exit procedures, and V2I commands.
Physically segregated lanes and road sections would allow the safe exploitation of all capabilities of fully automated mode, including centralised control via V2I commands as technical readiness permits. To implement such segregation, new road signs are needed that clearly mark the boundaries of these sections and inform all road users of the applicable rules.
Proposed Road Signs
New road signs, supplementing the Vienna Convention, are necessary to isolate autonomous vehicles from mixed traffic.
Cooperative autonomous vehicles are a more advanced category building on the capabilities of driverless vehicles (SAE L4/L5). All cooperative autonomous vehicles are fully driverless. In addition, they are equipped to share their real-time motion parameters (position, velocity, acceleration and planned trajectories) with the infrastructure and to execute centralised coordination V2I commands. These commands cover route selection, priority assignment when merging at lane drops or junctions, and dynamic headway management that takes into account the planned trajectories of surrounding vehicles. This continuous data exchange enables cooperative autonomous vehicles to operate safely with reduced gaps, negotiate right-of-way without traffic signals, and respond instantly to changes in the traffic environment communicated by the infrastructure. Consequently, cooperative autonomous vehicles achieve a higher level of traffic flow optimisation than isolated driverless vehicles can attain.
Because cooperative autonomous vehicles rely on a constant exchange of data with roadside equipment, their safe operation demands complementary infrastructure such as roadside units, dedicated short-range communication networks, and centralised traffic management platforms. This distinction in technical requirements is reflected in the proposed sign system, which introduces two separate zone signs. The Driverless (SAE L4/L5) vehicles zone sign marks a physically protected road space where any fully automated vehicle without a driver is permitted, regardless of its communication capabilities. The Cooperative autonomous vehicles zone sign designates an environment where only cooperative autonomous vehicles – those that can transmit their motion parameters and obey coordinated commands – are allowed, thereby enabling the full deployment of high-density, centrally managed traffic flow strategies.
The proposed road signs are listed in the table below.
Category | Design | Road signs with the symbol "A" | Road signs with the symbol "Vertical antenna with two waves on each side" |
|---|---|---|---|
Prohibitory or restrictive signs | White circle with a red border. Black symbol "A" in the centre. | No entry for driverless (SAE L4/L5) vehicles | n/a |
Special regulation signs | Vertical white rectangle. The word "ZONE" in black at the top. A blue circle in the centre containing a white symbol ("A" or antenna). | Driverless (SAE L4/L5) vehicles zone | Cooperative autonomous vehicles zone |
Special regulation signs | Vertical white rectangle. The word "ZONE" at the top. A grey circle with a white symbol ("A" or antenna) inside. Five black diagonal lines from lower left to upper right across the sign. | End of driverless (SAE L4/L5) vehicles zone | End of cooperative autonomous vehicles zone |
Special regulation signs | Blue square. White symbol ("A" or antenna) with a white arrow pointing left. | Beginning of lane for driverless (SAE L4/L5) vehicles to the left | Beginning of lane for cooperative autonomous vehicles to the left |
Special regulation signs | Blue square. White symbol ("A" or antenna) with a white arrow pointing right. | Beginning of lane for driverless (SAE L4/L5) vehicles to the right | Beginning of lane for cooperative autonomous vehicles to the right |
Information, facilities or service signs | Blue vertical rectangle. Inside a white square with a black symbol ("A" or antenna). | Stopping area for driverless (SAE L4/L5) vehicles | Stopping area for cooperative autonomous vehicles |
Additional panels | White rectangular panel. Black symbol ("A" or antenna) in the centre. | For driverless (SAE L4/L5) vehicles only | For cooperative autonomous vehicles only |
Additional panels | The word "EXCEPT" in black, followed by the black symbol ("A" or antenna). | Except for driverless (SAE L4/L5) vehicles | Except for cooperative autonomous vehicles |
Operational Use of the Proposed Signs
The design of each sign is specified in the table above. The signs are deployed at the boundaries where physically segregated infrastructure for autonomous vehicles connects to the conventional road network. Their primary purposes are to prevent accidental entry by human drivers, inform all road users of the special regulations in force, and enable distinct operational rules inside the segregated zone. Two complementary sign families exist: one for driverless vehicles, marked with the symbol "A", and one for cooperative autonomous vehicles, marked with the vertical antenna symbol.
1. Entry to and exit from a segregated zone
The boundary of any physically segregated road section or lane reserved for driverless vehicles or cooperative autonomous vehicles is indicated by the corresponding zone sign. This sign is placed at every entry point, near the physical barrier, such as a boom gate. It simultaneously grants access to the indicated vehicle category and prohibits entry for all other vehicles.
At every exit from the zone, the matching end-of-zone sign is installed. This sign informs departing vehicles that the special rules of the zone cease to apply.
2. Advance warning and entry at lane splits
When a segregated lane for driverless vehicles or cooperative autonomous vehicles branches away from a multi-lane road, advance warning is given by the sign Beginning of lane for driverless (SAE L4/L5) vehicles to the left (or to the right), or the equivalent sign for cooperative autonomous vehicles. It is placed before the divergence point. The actual entrance to the zone is marked by the zone sign itself, located at the point where the physically segregated infrastructure begins. Thus, the driver sees the advance warning sign first, then the zone sign at the entry.
3. Advance warning and entry at intersections
When a segregated road for driverless vehicles or cooperative autonomous vehicles begins at a junction and no separate lane is formed before the turn, advance warning is provided by the appropriate zone sign accompanied by an additional panel. The additional panel bears a black arrow (or multiple arrows) that indicates the permitted direction(s) of travel to reach the zone - left, right, straight ahead, or any combination of these. This combination is installed 50-100 m before the intersection. The same zone sign, without any additional panel, is placed at the legal boundary of the zone. Thus, the driver sees the zone sign with arrow panel first, then the zone sign after the turn or intersection.
4. Prohibiting access for autonomous vehicles
Where autonomous vehicles must be prohibited from entering an area - for example, for safety or security reasons - the prohibitory sign No entry for driverless (SAE L4/L5) vehicles is used.
No prohibitory sign is foreseen specifically for cooperative autonomous vehicles. Being a subset of driverless vehicles, they are already covered by the sign No entry for driverless (SAE L4/L5) vehicles.
5. Service and supplementary information
Inside a segregated zone, the information sign Stopping area for driverless (SAE L4/L5) vehicles (or Stopping area for cooperative autonomous vehicles) marks designated locations for passenger pick‑up and drop‑off.
Two additional panels complete the system.
The panel For driverless (SAE L4/L5) vehicles only (or For cooperative autonomous vehicles only) can be mounted below other regulatory or information signs to restrict their application to the indicated vehicle category. For instance, when placed under a standard parking sign, it denotes a parking place reserved solely for autonomous vehicles.
The panel Except for driverless (SAE L4/L5) vehicles (or Except for cooperative autonomous vehicles) can be mounted below other regulatory or information signs to exclude their application to the indicated vehicle category.
Conclusion
The proposed signs provide a complete and internationally compatible system for marking physically segregated autonomous-vehicle infrastructure. By addressing both advance warning and zone demarcation, and by distinguishing between driverless and cooperative vehicles, the system enables the safe, gradual deployment of high-density automated traffic flows while keeping human drivers fully informed. Its adoption would remove a key regulatory barrier currently preventing driverless vehicles from reaching their full potential.
