R1

R1 / 1:M

Road splits across intersections into one or many Road Segment

Road Segment is stretch between intersections in one Road

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Picture the driver’s perspective looking out the front windshield of the Ego Vehicle. The Road lies ahead with clear left and right boundaries. Now we see another Road up ahead crossing ours at an oblique angle. This Road also has left and right boundaries, but from what perspective?

As always, we take the Ego Vehicle perspective. So, if we were to turn at the next intersection, onto this angled road, how would the boundaries of that road align? If we turn right, assuming two way crossing traffic, we have the nearest boundary on our right and the furthest on the left. Turning left, just the opposite.

If we approach an intersection and continue straight through, remaining on the same map road (CA-82) let’s say, we enter a new Road Segment, but retain the same alignment with respect to the Ego Vehicle. So, re-orientation may or may not result in a new alignment of left/right boundaries.

Normally, a map road will continue straight through an Intersection. But many times continuing straight through an Intersection puts the Ego Vehicle on a different Road. To continue on the same Road, it may be necessary to take a left or right turn at the Intersection.

For both potential re-alignment of left/right boundaries and establishing what actual map Road the Ego Vehicle is on, it is helpful to abstract the notion of Road Segments.

In the case where an Ego Vehicle is traveling straight ahead on a single path of asphalt with no parallel or crossing paths, the Ego Vehicle is on a single Road consisting of only one Road Segment.

When a crossing road is observed connecting at an Intersection ahead, a Road Segment is seen on each interface to the Intersection. If traveling straight ahead, through the Intersection, the Ego Vehicle will leave one Road Segment and enter another on the other side. These two Road Segments may or may not belong to the same Road. They won’t if the map road turns left or right instead of continuing straight.

A Road Segment is always part of a single Road. If, for some reason, map information is unavailable, temporary assumptions can be made about which Road a Road Segment belongs to.

In the world of maps, roads exist all the time, but from the Ego Vehicle’s perspective, a Road is defined by observable Road Segments and cannot exist without at least one.

Formalization

Road Segment.Road -> Road.Name