In this article we present a recommended best practice for numbering the lane objects in your intersection (the instances of Lane Types which make up the intersection). Please read the prior article on numbering approaches.
Part II: Numbering the Lane Objects
The prior article discusses how to consider, organize, and rotate each approach before starting.
Take away: The first master rule is to always assign a unique number for every possible lane object there could ever be in the intersection. By following this process, the lane number assignments you make will be fixed, so that along with the region and the intersection ID, they will form a world-wide unique value that never changes as the intersection is maintained The second master rule is to simply assign values from one side moving to the other.
Number the lanes in each approach in turn. Begin by rotating the approach so that the stop line is at the bottom and the inbound (ingress) lanes are facing below. In the representative diagram below it is easy to see the different lanes types used in the section dealing with Approach numbering.
As discussed here, the term “approach” takes on two flavors in a DSRC MAP intersection. The Ingress approach and the Egress approach are each given their own number (when they are modeled at all). These approaches are commonly made up of the motor vehicle lane types. This leads to the common question of into which approach the other lane types (crosswalks, bike lanes, medians, etc.) best fit. It really does not matter, as the approach selected is not part of the overall indexing system. In other words, the Ingress and Egress approaches are not material to the Lane numbering assignment. Not all Lane Types need to be part of an approach (median lanes being a good example of this).
This part of the exercise is mostly a mental effort to organize your approach to the work.
Here we have used a simple system of grouping the sidewalks (and any bike lane(s) – none are shown in the above) as well as any median present all into the Ingress Approach. The Egress Approach then holds the outbound motor vehicle lanes, bike lanes and sidewalks. As described elsewhere, some prefer to place the tangential crosswalk into a special approach with other crosswalks, while others group it with the Ingress Approach. In either case, it is numbered along with other lanes in the approach.
One can now see all the different lanes in this example approach. In the above image we label them for future reference. Note that the above does not show Bike Lanes (or Bike Boxes) for simplicity.
Numbering these Lanes from left to right as shown produces twelve lanes numbered from “0xN+0” to “0xN+b” (or 0xN+11 if you prefer a decimal expression). Note that four index values in the approach remain unassigned. [The vast majority of intersections fit neatly into this numbering scheme]
Recall that a best practices is to define a ranges of Lane ID numbers can be assigned to each approach. Ranges of 16 and 32 have been suggested, based on the geometry of the intersection. Recall also that the Lane ID is a binary value used to link to the lane, so consistently displaying it in base 16 (Hex) is also of value at times. In the above we have used “N” to indicate which range these lanes are a part of.
This numbering pattern also allows some simple decoding of the lane number when humans have to decode and read the actual MAP and SAP messages. If the first approach due is North, the very north- and westmost feature (either a sidewalk or the right most lane) is likely to be labeled 0x10. If this is a typical four-approach intersection, the Southeast-most lane feature will be numbered 0x30, etc.
Worked Example, The Reference Intersection
Below are the Lane numbers and Approach numbers for the reference intersection used as an example in these materials.
to be provided – image of numbered approaches
to be provided – image of numbered lanes
See Also: The hand drawn map example where this numbering plan is used.