Express and Service Lane support in routing and pathfinding

[Elesbaan70]

Description

Some cities such as New York, Melbourne, and Buenos Aires, have city streets with three, four, or six carriageways. The outer carriageways are "service roads" which provide access to homes and businesses, parking, bicycle lanes, etc., while the inner carriageways are "express roads" with fewer conflicts and hazards. Unlike a limited access highway, these roads have at-grade intersections, though New York uses grade-separated interchanges in some cases.

Similarly, Airports DLC has a large cargo road which functionally has four carriageways, separated by flush medians. The express/service road terminology does not apply in this case, but these terms will still be used since they are clear and apply to most use cases.

This road design is not to be confused with "frontage roads," which are two-way roads running alongside a major artery. The terminology varies regionally, but this issue should be discussed using consistent terminology to avoid confusion.

Currently there are two limitations in the design of CS and TMPE which are impediments to functional roads of this kind:

1. The game has no true awareness of medians. The apparent support for medians on basic roads is actually a hack where a road asset is flagged to prevent cars from crossing oncoming traffic to make a stop. This means that there is no straightforward way to completely prevent vehicles from crossing a median. Even the Lane Connector tool is ignored by emergency vehicles.
2. There is no reliable way to route thru traffic onto the express lanes. A significant difference in speed limit between express and service lanes will help, but not truly resolve this issue.

Definitions

Express Lane - On a road without displaced lanes, on which where standard (non-displaced) lanes* in the same direction are separated by a median, an Express Lane is a lane that is on the left side (right-hand traffic) or right side (left-hand traffic) of that median.

Service Lane - On a road without displaced lanes, on which where standard (non-displaced) lanes* in the same direction are separated by a median, a Service Lane is a lane that is on the right side (right-hand traffic) or left side (left-hand traffic) of that median.

* In a multi-carriageway road, displaced lanes are presumed to be special-use and are not considered candidates for the service/express lane configuration. For example, Avenida 9 de Julio in Buenos Aires.

Median Detection

For the purposes of express and service lanes, certain lanes will be understood to indicate the presence of a median. A lane will be presumed to indicate a median if it meets the following criteria:

1. The Can Cross Lanes setting is unchecked. (This is functionally a median flag, though it is a hack.)
2. The lane is not a vehicle lane.
3. The nearest vehicle lanes on either side of it are standard (non-displaced) lanes going the same direction.
4. One of the following conditions are met:
   1. The lane is at a higher vertical offset than that of the vehicle lanes around it, but with a difference not exceeding 4 meters (raised median), or
   2. The gap between the nearest vehicle lanes on either side is at least as wide as the widest of those lanes (flush median).
5. The lane does not overlap any vehicle lane.
6. For each direction there is no more than one lane meeting these criteria. However, multiple lanes meeting these criteria may be considered as one for this purpose if they are adjacent with no vehicle lanes inbetween.

It is unclear whether any road workshop assets currently exist that would benefit from this feature. If there are, but the above criteria are too restrictive, an "Aggressive Median Detection" option could be added to loosen the median lane requirements, and possibly even act on other more ambiguous criteria such as wide gaps between lanes.

Express and Service Lane Detection

Express and Service Lane detection will be an enhancement to the NetInfo extensions currently in development for Displaced Lane Detection.

Routing

For all nodes with only two segments, where express and service lanes are present on both segments, lane transitions that cross between express and service lanes will be forbidden unless explicitly enabled using the Lane Connector. Relaxed transitions will be allowed for flush medians, but forbidden for raised medians.

Pathfinding

The pathfinding logic will mathematically apply a penalty to the use of service lanes, so that vehicles will tend not to use them except at the start or end of a trip.

With this issue, the convention is established that express lanes will have the Allow Connect flag unchecked. The pathfinding logic will be updated to mathematically reward the use of lanes without this flag when both are available on the same road, so that vehicles will tend to avoid service lanes except at the start or end of a trip.

The Airports DLC cargo roads leave this flag checked on all lanes, and other roads for similar use cases should do likewise.

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Real-World Examples

Queens Boulevard, New York

Four carriageways, one-way slip lanes

Grand Concourse, New York

Three carriageways, two-way center, one-way slip lanes

Springvale Road, suburban Melbourne

Four carriageways, two-way slip lanes, turn lanes to access service roads on opposite side

Avenida 9 de Julio, Buenos Aires

Six carriageways, center pair are inverted (displaced) bus lanes, with pedestrian lane and bus stops in the center median

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Possible Future Enhancements

Disable Traffic Lights by default on intersections where the Lane Connectors or Arrows allow access only to the service roads.

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Group Traffic Lights for Express and Service Lanes like the Dislaced Lanes grouping currently in development

There is no known use case for this, but if any are identified, it would probably be an easy enhancement.

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Traffic Lights Per Lane

This would allow better traffic management at the slip lanes in certain situations.

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Bicycle Lane Traffic Lights

In conjunction with traffic lights per lane, bicycle lane traffic lights would allow the creation of crossings like this, which are common in New York:

[Elesbaan70]

Added real-world examples and noted the existence of six-carriageway roads. Changed definitions and median detection rules to allow for use of displaced lanes on the same road as express/service lanes (which must always be standard lanes).

[Elesbaan70]

Added Airports DLC cargo road support to this issue by way of adding the recognition of flush medians, and providing a specific mechanism for telling pathfinding to show preference for service lanes.