Map model details

A/B Street builds a rich representation of a city map using OpenStreetMap (OSM) and other sources. This chapter describes how.

TODO: Integrate pictures from these slides.

This recorded presentation covers some of this.

The map

A single city is broken down into different pieces...

A/B Street comes with a few maps, each defined by a bounding/clipping polygon for some portion of Seattle. Each map has these objects:

  • Roads: A single road connects two intersections, carrying OSM metadata and containing some child lanes.
  • Lanes: An individual lane of traffic. Driving (any vehicle), bus-only, and bike-only lanes have a direction. On-street parking lanes don't allow any movement, and they have some number of parking spots. Sidewalks are bidirectional.
  • Intersections: An intersection has references to all of the incoming and outgoing lanes. Most intersections have a stop sign or traffic signal policy controlling movement through it.
    • Border intersections on the edge of the map are special places where agents may appear or disappear.
  • Turns: A turn connects one lane to another, via some intersection. (Sidewalks are bidirectional, so specifying the intersection is necessary to distinguish crosswalks at each end of a sidewalk.)
  • Buildings: A building has a position, OSM metadata, and a front path connecting the edge of the building to the nearest sidewalk. Most trips in A/B Street begin and end at buildings. Some buildings also contain a number of off-street parking spots.
  • Area: An area has geometry and OSM metadata and represents a body of water, forest, park, etc. They're just used for drawing.
  • Bus stop: A bus stop is placed some distance along a sidewalk, with a pointer to the position on the adjacent driving or bus lane where a bus stops for pick-up.
  • Bus route: A bus route has a name and a list of stops that buses will cycle between. In the future, they'll include information about the frequency/schedule of the route.
  • Parking lot: A parking lot is connected to a road, has a shape, and has some internal driving "aisles." The number and position of individual parking spots is auto-generated.

Coordinate system

A/B Street converts (longitude, latitude) coordinates into a simpler form.

  • An (x, y) point starts with the top-left of the bounding polygon as the origin. Note this is screen drawing order, not a Cartesian plane (with Y increasing upwards) -- so angle calculations account for this.
  • The (x, y) values are f64's trimmed to a few decimal places, with way more precision than is really needed. These might become actual fixed-point integers later, but for now, a Pt2D skirts around Rust's limits on f64's by guaranteeing no NaN's or infinities and thus providing the full Eq trait.
  • A few places in map conversion compare points using different thresholds, usually below 1 meter. Ideally these epsilon comparisons could be eliminated in favor of a fixed-point integer representation, but for now, explicit thresholds are useful.


Ideally, the finalized maps would satisfy a list of invariants, simplifying the traffic simulation and drawing code built on top. But the input data is quite messy and for now, most of these aren't quite guaranteed to be true.

  • Some minimum length for lanes and turns. Very small lanes can't be drawn, tend to break intersection polygons, and may lead to gridlocked traffic.
  • Some guarantees that positions along adjacent lanes actually match up, even though different lanes on the same road may have different lengths. Examples include the position of a bus stop on the sidewalk and bus lane matching up.
    • Additionally, parking lanes without an adjacent driving lane or bus stops without any driving or bus lanes make no sense and should never occur.
  • Connectivity -- any sidewalk should be reachable from any other, and most driving lanes should be accessible from any others. There are exceptions due to border intersections -- if a car spawns on a highway along the border of the map, it may be forced to disappear on the opposite border of the map, if the highway happens to not have any exits within the map boundary.


For a single mode, each lane is connected to two intersections. Turns connect two lanes. There are no turns between sidewalks and driving/bike/bus lanes.

All buildings and parking lots have driveways. This must connect to a sidewalk, allowing pedestrians to enter/exit that object. The driveway OPTIONALLY connects to the nearest driveable lane. This allows cars to enter/exit that object for parking.

Public transit stops are located somewhere on a sidewalk. They're associated with a driveable position where the bus or train stops. In the future, this will need to account for dedicated surface-level platforms and for underground transit stations, likely associated with a building.

There's a concept of "parking blackholes." If you treat every road as bidirectional without access restrictions, then the graph is connected. But the more detailed view has to factor in one-way roads and things near the map border. These blackholes influence where cars will try to look for parking (since we don't want them entering a blackhole and getting stuck) and also, for temporary/unintentional reasons, where pedestrian<->bicycle transitions will happen.