In the state of Michigan it is common to extend the left turn lane past the intersection and make it an effective u-turn lane. Sometimes these lane have a signal controller and a phase state, while at other times they are simply un-signalized with a yield to on-coming traffic. Both one and two lane variations are used.
Article is Very Draft still,
is pending the improved polygon rendering software
to complete showing the width editing issues.
You can read more about this interesting local convention in these articles:
- https://en.wikipedia.org/wiki/Michigan_left and also:
- And here is a nice users guide for those of us from out of town.
How to encode a Michigan Left in MAP-SPaT work
In this example we consider modeling the turn lanes found on the Western side of an intersection located at 16 Mile Road and Van Dyke in the city of Sterling Heights, Michigan.
In the image below, two such turns can be seen, displayed in two ways. On the left is a raster image of the roadway, on the right is a stylized image from open street maps. These two image are precisely aligned (we use *prj and *pgw files for this). Note how the open street map takes some liberties with the physical layout of the lanes.
Repeating this map, but with the GIS control data created below we have:
For this example we will consider only the turn on the Western (left) side. Again, repeating this map as a close-up, but with the GIS data created below we have:
Aside: The black colored curve that appear in these maps represents the 300 meter radius lines to the center of the intersection with Van Dyke to the East on this road.
Initial Capture Process
The overall process is similar to any other DSRC MAP capture. Given a suitable (and registered) base map, first:
- Various stop lines are established (this particular lane is not signalized) and then
- Various control line are established (curb edges, stripes, crosswalks etc.)
Then the center lines of the desired lanes are captured (in a full 32bit LLH systems, often in KML or some other precise methods supported by the GIS tool).
The resulting center line, in conjunction with its bounding curbs, can now be used for further editing to add the lane widths and adjust the offset points that are to be used.
This general process is described further in the articles found here (tbd link).
To be created.
Adding lane widths to a curved section of road in the absence of a visual tool is particularity difficult. This arises from the inability of the user to correctly visualize the tangential width lines projected between each of the two offset segments. It often also involves the movement of the offset point to better reflect the true lane center line.
Add edited image here
The resulting polygon
To be created.
This section is pending the improved polygon rendering software to complete showing the width editing issues.
The 2nd Lane
Many Michigan Lefts are in fact two lanes. The general idea is that the vehicles in the left hand lane will proceed along with traffic, while the right hand lane is used to cut oncoming across traffic to reach the rightmost lane (and thereby to complete the original maneuver which was to turn left at the prior intersection).
We now consider options regarding how to model the presence of both lanes.
To be created.
The Case for a Curve
This example also serves to demonstrate the growing need for a curve map primitive so that the curvature need not be represented by a set of straight line offsets. This is a topic area for improvement that the DSRC TC has yet to resolve (it is part of the unfunded 2018 plan).
In the case of the Michigan Left, and unlike the more typical roadway curves found for freeway exist ramps and for curve speed warning uses in general, there is no apparent need for the final segment of the generic lane to have a small tangential angle value at the final point. Several basic curve modeling types appear to be suitable for use in this regard.