In this article we present a recommended best practice for organizing and numbering the approaches in your intersection. Numbering the Lane Objects (the instances of Lane Types which make up the intersection) is then considered.
Take away: The master rule is to simply number approaches from the north-most approach in a counter-clock wise order, then adding other lane objects into approaches as/when needed. In each resulting approach, number ALL possible lane objects (proceeding from on side to the other) that may someday exist regardless of whether those items will be captured into the map at this time.
The values of the the LaneIDs you assign should be permanent for several reasons, this is discussed further here <link>. This guidance below is as simply a best practice to get that done will less rework and effort when a change occurs.
Note: Read this article <add link> on the different types of approaches used in the MAP work and how these both relate and differ from the common term approach used in traffic engineering.
We begin with a simple plan for approach organization and an approach numbering system to establish groups of lanes which can be worked as a complete set. This allows the map creator to establish a range of lane indexing values assigned to all possible lanes in the intersection approach. Even if the lane in question will not be captured and/or added to the final map at the time of creation.
A Basic but Typical Intersection
A basic, but typical, Intersection contains four approaches (North, East, South, and West) and can be represented as shown below.
It is obvious that there are four approaches from a typical traffic engineer’s point of view. From a MAP point of view there are in fact eight as the MAP differentiates between in-bound (ingress) movements and out-bound (egress) movements. The detail becomes of consequence as the lane details are further developed, so it is mentioned here.
Aside: In the above image we shown traffic moving on the right hand side with small arrows, as is customary in the US. But the same rules presented here (as well as the MAP structures in general) works equally well in the 185+ countries which drive on the left side of the road.
A common variation of this is to also include pedestrian crosswalks and selected sidewalk features in the map as well. What approach to place these object into is considered in a moment.
Part I: Numbering the Approaches
- The master rule is to simply number approaches from the north-most approach in a counter-clock wise order, then adding other object into approaches as/when needed.
The determination of what is north-most can at time be a judgment call, simply pick something and move on. In a simple case like this 4-way intersection it is obvious.
Within in each of these approaches we will assign LaneID values (from within an assigned range) for any and all possible lane objects. Start from one side (left) to the other. [As will be seen; we recommend rotating each approach before the numbering]
The guidelines for the assignment of the LaneId to the lane object in each approach will be covered in Part II
The LaneID data element allows up 254 possible assignments per intersection, so it is very unlikely you will run out. If you have less then 6 approaches and have many lanes in each (say >15), then number lanes in each approach in blocks of 32. Otherwise use blocks of 16. Recall that these value can be arbitrarily assigned if these sizing rules do not fit a complex intersection.
We now need to consider how to best group the crosswalk lanes. There are two schools of thought on this, both seem equally well suitable so we provide no strong recommendation either way. You can:
- Add each crosswalk to the same approach and numbering range as the lanes that spans in along side of it.
- Or, Add the each crosswalk lane to an approach dedicated to crosswalk lanes with it own numbering range.
If you elect to do the later style, you must then gather and number all the crosswalk lanes into an approach of their own. In the simple 4-way example above this then become a new 5th approach
Note that we have not considered diagonal crosswalks spanning the middle of the intersection (often called pedestrian scrambles). Using a dedicated approach (for either these diagonal lanes or all crosswalk lanes) seems to be preferable in these cases.
Intersections with Lane Storage
Intersections which have Lane Storage do not fit into the general rule to number the approaches in a counter clock-wise fashion. So an additional rule is presented, to number these inner lanes from North to South after the counter clock-wise rule has been used.
So in the example above the storage lanes would be approach #5. If a crosswalk approach was then to be added, it would be #6.
In some intersections there are additional important lane details such as a train and tracks crossing near the intersection, or tracks crossing directly thought the intersection no mans land itself. This article on rail crossings (called tracked vehicle lanes) has images of two such layouts near SCSC offices.
In this case the guidance is to add the additional approaches after all the roadway approaches and any approaches added to service needs for crosswalks, dedicated bike lanes, and other users types. In other words this should always be the a last approach.
Part II: Numbering the Lane Objects
Now moved to be its own article.
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: This article on pedestrian crosswalk lane numbering practices.
See Also: The hand drawn map example where this numbering plan is used.