Instruction Manual: How to pick the center of an intersection

Rules of thumb:

  • At non-intersections (such as hidden shaping points), position the waypoint on the centerline of the highway.
  • Usually position the waypoint at the point where the centerlines of the two highways cross. Often the same coordinates can be used for both highways.
  • These centerlines are defined by through lanes, not by turning lanes at intersections nor the equivalent in interchange ramps.
  • For couplets and divided highways, usually split the difference between the two roadways.
  • There are common exceptions to positioning at centerline crossings, such as interchanges where ramps connect nearby, non-intersecting highways, or where a short access road connects a road to another with a trumpet or similar interchange. In these cases, the waypoints for the same interchange on the separate highways cannot be at the same coordinates. Instead, the waypoints should be where the connecting ramps or access road interchange with each highway.
  • Designations of the roadways are irrelevant. The physical configuration of the junction is what matters.
    As an example, imagine a cloverleaf interchange where one loop ramp was replaced by a flyover ramp. Consider the I-97 & MD 3 & MD 32 interchange.

    The point should be centered in the middle, and the same point should be used for I-97, MD 3, and MD 32.. The centerlines are between the NW-SE and the NE-SW through lanes. One might have expected the point for I-97's file to be on the apparent SE-NE mainline, or even two points where MD 32 connects at the SE and MD 3 at the NE. But it is one interchange, and the point for all three routes should go in the middle.
  • Some interchanges have unusual shapes or are stretched-out versions of normal interchanges. Use your best judgment.

Interchanges

  • Diamond interchange: where the centerlines cross.


  • Double half interchanges: Usually use one central point and treat both halves as a single, full interchange. Exceptions: a clear gap of at least 0.5 mi/0.8 km separates the two halves, or each half connects to a different highway that we are also mapping.
  • Partial cloverleaf interchanges: usually where the centerlines cross.


  • Misbehaving diamond/partial cloverleaf interchanges: some cases are better handled by putting the point where the ramps connect to the freeway. In this case, the points on the freeway and the cross road will not line up.
  • Cloverleaf interchanges: where the centerlines cross.

  • Trumpet interchanges: where the access road centerline crosses the freeway centerline. Double trumpet interchanges get separate points at each trumpet.

  • 2-way high-speed interchanges: where the centerlines would cross if it were an at-grade intersection with the same shape. Not where the ramps of one road connect to the other.

  • 3-way high-speed interchanges: in the middle of the central ramp triangle, not necessarily on a ramp.

 

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