Lane Model

The Lane Model includes four major components:

  • Lane Topology: the width of a road, the number of lanes, and the mapping of lanes through intersections, merges, and splits.
  • Lane Geometry: the shapes of lanes, and the positions of their boundaries.
  • Lane-Road References: the relationships between the Lane Model and the Road Model.
  • Lane Attributes: the attribution applied to lanes and lane boundaries, such as a bus-only lane or turn lane.

For details, see the subsections below:

Lane topology

Note: The terms used below (accessors, portals, and surface paths) are not industry standard. Some companies might use different terms for similar concepts.

In the simplest case, a road in the Lane Model has a consistent lane topology that's defined by accessors and portals. In the figure below, accessors are the cyan lines perpendicular to the road, and portals are the yellow triangles pointing in the direction of travel for each lane on the road.

Lanes, accessors, and portals
Figure 1. Lanes, accessors, and portals
  • Accessors define the 3D locations of roads, as shown in the figure above. An accessor is a vector in 3D space. The endpoints of an accessor are geo-located — typically with latitude, longitude, and elevation in meters relative to sea level of the applied ellipsoid. Whenever a road changes the number of its lanes, this change in captured in the map by placing a new accessor on the road where this change begins and placing another accessor where the change is complete.

  • Portals define the number of lanes on a road and the travel direction of each lane. Portals are separated by parametric points along accessors that define the relative widths of lanes, without gaps or overlap. This relative width is on a parametric scale of 0-1 (i.e. 0.0 and 1.0 are always at the start and end of an accessor); it's not a measure of physical distance. As illustrated in the example below, a road with two lanes in each direction, all of equal relative width, has portal separators at 0.25, 0.5, and 0.75.

Figure 2. Portals

Lane connectivity, which defines each legal and logical path that traffic can take, is another key consideration of lane topology. In the simple case of a road that adds or removes lanes, the lane inputs are mapped to lane outputs as shown by the dashed blue lines in the figure below.

Lane connectivity - add lane
Figure 3. Lane connectivity - add lane

More complex examples of lane connectivity arise at intersections, which are defined as the convergence of three or more roads. In the figure below, the dashed lines show the lane mappings, and the portal arrows show directionality. Note that an accessor is placed where each road surface meets the intersection.

Lane connectivity - intersection
Figure 4. Lane connectivity - intersection

Motorway junctions, such as off-ramps, are treated like a type of intersection (as shown below). Lane mappings follow each logical and legal path that traffic can take. Note that an accessor is placed where the off-ramp starts forming, and another accessor is placed where the formation is complete.

Lane connectivity - off-ramp
Figure 5. Lane connectivity - off-ramp

Lane geometry

In the Lane Model, roads are surfaces in 3D space. The physical width of roads is defined by the geo-locations of accessor start/end points, which in turn defines the boundaries of the road.

The bearing, curvature, and slope of road surfaces are defined by surface paths, which are 3D NURBS (non-linear uniform rational B-splines) that start and end at parametric points on accessors and run along the center of road surfaces and through intersections where traffic flows. The curvature of a road surface is derived from the geolocated endpoints of accessors and control points that determine the shape of surface paths. As the figure below shows, the surface paths (green splines with arrows) on either side of the same accessor can be offset, but they must have the same tangent and curvature where they meet at an accessor to ensure smooth continuity in the shape of the road.

Surface paths
Figure 6. Surface paths

In the figure below, the arrows at the ends of green surface paths point in arbitrary directions, unrelated to the travel direction of traffic. The 3D curvature of surface paths is shaped by geolocated control points somewhat like a 3D version of Bezier curves that are common in 2D vector art.

Control points for surface paths
Figure 7. Control points for surface paths

Lane path geometry and lane boundary geometry are also derived from accessors, portals, and surface paths as shown below.

Derived lanes
Figure 8. Derived lanes

Lane-Road references

Map data in the Lane Model is seeded from road topology and lane-related data in the Road Model. The Lane Model tracks the corresponding topology segments and nodes in the Road Model to keep map data in the two models in sync. Specifically, the surface paths in the Lane Model refer parametrically to segments (links) identified by unique IDs in the Road Model, as illustrated below.

References between Lane Model and Road Model
Figure 9. References between Lane Model and Road Model

Lane attribution

The following attributes are commonly applied to lanes:

  • Lane Type — including Regular, Acceleration, Deceleration, Turn Lane (right, left, U-turn), Shoulder, etc.
  • Lane-Level Speed Limit for cases in which a lane has a different speed limit than the parent road.
  • Lane Access Characteristics for cases when access to a lane differs from access to the parent road. Examples: HOV lane, truck-only lane, bus-only lane, etc.
  • Lane Transition Status: forming and ending. (Note: We can derive whether a lane is forming or ending based on the travel direction coupled with the topology.)

The following attributes are commonly applied to lane boundaries:

  • Lane Boundary Markings — including Style (solid, dashed, etc.), Color (white, yellow, etc.), Material (paint, mechanical such as Botts Dots, etc.), Width (regional standards or custom), and Lateral Offset. (Note: In practice, Lateral Offset is handled using a limited number of styles that represent multiple-stripe boundaries: double solid, double dashed, solid-dashed, dashed-solid.)
  • Lane Boundary Traversal: Can you change lanes? Yes or no left-to-right, right-to-left, or both ways.

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