On Placing Stoplines

This article deals with the mechanical details of where to place your “control” stop lines. Drawing the stop line, as well as many other control lines is preliminary step before beginning to capture the lane center lines when creating a map. This overall process is discussed further in this (to be provided) article.

We presume here that you are using a “real” GIS tool where the zoom displayed by the tool is not limited by the finite resolution of any underlying raster images.  And that the line you will draw is mathematically thin (e.g. it is not limited by display pixels). This is a very important ability that allows the MAP developer to use the power of the human eye to perform a low-pass filter and precisely detect the edges of the image and to then place the control lines with high accuracy.

Best Practices:

As a best practice, place the control line directly in the center of the painted stop line area.

Do not be concerned with the width of the painted stop line.  Unless your agency has developed an alternative policy, do not skew the control line to the front or back of the line edge.  There is some argument that the control line should be placed at the leading edge (in the direction of travel) of the stop bar.  But at this point in time there is insufficient practical experience to state this has value, while it does slightly increase map capture costs.

Start the lane offset points from a point along the control line and proceeding away from the intersection. Note that the first offset is from the intersection anchor point to this point.

It is also a best practice to establish control stop line for the egress lanes.  These are not shown here.

When a pedestrian crosswalk lane abuts the stop line (which may be coincident with the edge of the lane), it in common to ensure that the width of the pedestrian crosswalk does not intrude into the painted stop line.

A Worked Example

Consider the intersection below where there are several stop lines, including some stop lines associated with the storage lanes.   The intersection used here is located at 16 Mile Road and Van Dyke in the city of Sterling Heights, Michigan.

Google Map Image

When the stop lines have been added we have the below. Note that the top-left stop line (the South bound lanes) is not clearly evident and may required field validation to confirm.
Hint: Grab the slider bar and draft left or right to see both images.

 

Getting it Just Right

At the above image scale it all looks quite nice and pretty, but when zoomed into to a large degree the pixilation effects are more evident.  You need to set your control line with this type of zoom and having the line “equally split” the pixilated edge which you seek to capture.

The ability to perform precise placement control over the control line is more evident. This in turn allow obtaining very clean lane points.

 

In General

This general methodology should be followed for any control or center lines taken from raster image sources.  You can see this applied to the below close up of a intersection corner taken from the intersection of on Dixon Landing Road in Milpitas, California, USA.

 

Keep in in mind that some raster images are very well registered, typically they provide projection files as well. Others (notably Goggle) can be of varying quality with adjacent map tiles offset by noticeable amounts.  The user must remain on guard for that occurrence.

Map Conventions

The line styles used here follow SCSC internal MAP-SPAT conventions are are further described here (TBD link at this point).

 

 

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