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<ol class="chapter"><li class="chapter-item expanded "><a href="../index.html"><strong aria-hidden="true">1.</strong> Overview</a></li><li class="chapter-item expanded "><a href="../howto/index.html"><strong aria-hidden="true">2.</strong> Instructions</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../howto/new_city.html"><strong aria-hidden="true">2.1.</strong> Importing a new city</a></li></ol></li><li class="chapter-item expanded "><a href="../how_it_works.html"><strong aria-hidden="true">3.</strong> How it works</a></li><li class="chapter-item expanded "><a href="../case_studies/index.html"><strong aria-hidden="true">4.</strong> Case studies</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../case_studies/lake_wash.html"><strong aria-hidden="true">4.1.</strong> Lake Washington Blvd Stay Healthy Street</a></li><li class="chapter-item expanded "><a href="../case_studies/west_seattle.html"><strong aria-hidden="true">4.2.</strong> West Seattle mitigations</a></li><li class="spacer"></li></ol></li><li class="chapter-item expanded "><a href="../dev/index.html"><strong aria-hidden="true">5.</strong> Developer guide</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../dev/misc_tricks.html"><strong aria-hidden="true">5.1.</strong> Misc developer tricks</a></li><li class="chapter-item expanded "><a href="../dev/api.html"><strong aria-hidden="true">5.2.</strong> API</a></li><li class="chapter-item expanded "><a href="../dev/testing.html"><strong aria-hidden="true">5.3.</strong> Testing</a></li><li class="chapter-item expanded "><a href="../dev/mass_import.html"><strong aria-hidden="true">5.4.</strong> Importing many maps</a></li><li class="chapter-item expanded "><a href="../dev/data.html"><strong aria-hidden="true">5.5.</strong> Data organization</a></li></ol></li><li class="chapter-item expanded "><a href="../map/index.html" class="active"><strong aria-hidden="true">6.</strong> Map model</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../map/details.html"><strong aria-hidden="true">6.1.</strong> Details</a></li><li class="chapter-item expanded "><a href="../map/importing/index.html"><strong aria-hidden="true">6.2.</strong> Importing</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../map/importing/convert_osm.html"><strong aria-hidden="true">6.2.1.</strong> convert_osm</a></li><li class="chapter-item expanded "><a href="../map/importing/geometry.html"><strong aria-hidden="true">6.2.2.</strong> Road/intersection geometry</a></li><li class="chapter-item expanded "><a href="../map/importing/rest.html"><strong aria-hidden="true">6.2.3.</strong> The rest</a></li><li class="chapter-item expanded "><a href="../map/importing/misc.html"><strong aria-hidden="true">6.2.4.</strong> Misc</a></li></ol></li><li class="chapter-item expanded "><a href="../map/edits.html"><strong aria-hidden="true">6.3.</strong> Live edits</a></li><li class="chapter-item expanded "><a href="../map/platform.html"><strong aria-hidden="true">6.4.</strong> Exporting</a></li></ol></li><li class="chapter-item expanded "><a href="../trafficsim/index.html"><strong aria-hidden="true">7.</strong> Traffic simulation</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trafficsim/discrete_event.html"><strong aria-hidden="true">7.1.</strong> Discrete event simulation</a></li><li class="chapter-item expanded "><a href="../trafficsim/travel_demand.html"><strong aria-hidden="true">7.2.</strong> Travel demand</a></li><li class="chapter-item expanded "><a href="../trafficsim/gridlock.html"><strong aria-hidden="true">7.3.</strong> Gridlock</a></li><li class="chapter-item expanded "><a href="../trafficsim/trips.html"><strong aria-hidden="true">7.4.</strong> Multi-modal trips</a></li><li class="chapter-item expanded "><a href="../trafficsim/live_edits.html"><strong aria-hidden="true">7.5.</strong> Live edits</a></li><li class="chapter-item expanded "><a href="../trafficsim/parking.html"><strong aria-hidden="true">7.6.</strong> Parking</a></li><li class="spacer"></li></ol></li><li class="chapter-item expanded "><a href="../project/index.html"><strong aria-hidden="true">8.</strong> Project</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../project/roadmap.html"><strong aria-hidden="true">8.1.</strong> Roadmap</a></li><li class="chapter-item expanded "><a href="../project/motivations.html"><strong aria-hidden="true">8.2.</strong> Motivations</a></li><li class="chapter-item expanded "><a href="../project/history/index.html"><strong aria-hidden="true">8.3.</strong> History</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../project/history/backstory.html"><strong aria-hidden="true">8.3.1.</strong> Backstory</a></li><li class="chapter-item expanded "><a href="../project/history/year1.html"><strong aria-hidden="true">8.3.2.</strong> Year 1</a></li><li class="chapter-item expanded "><a href="../project/history/year2.html"><strong aria-hidden="true">8.3.3.</strong> Year 2</a></li><li class="chapter-item expanded "><a href="../project/history/year3.html"><strong aria-hidden="true">8.3.4.</strong> Year 3</a></li></ol></li><li class="chapter-item expanded "><a href="../project/CHANGELOG.html"><strong aria-hidden="true">8.4.</strong> Full CHANGELOG</a></li><li class="chapter-item expanded "><a href="../project/references.html"><strong aria-hidden="true">8.5.</strong> References</a></li><li class="chapter-item expanded "><a href="../project/collaborations.html"><strong aria-hidden="true">8.6.</strong> Collaborations</a></li></ol></li><li class="chapter-item expanded "><a href="../side_projects/index.html"><strong aria-hidden="true">9.</strong> Side projects</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../side_projects/parking_mapper.html"><strong aria-hidden="true">9.1.</strong> Mapping on-street parking</a></li><li class="chapter-item expanded "><a href="../side_projects/osm_viewer.html"><strong aria-hidden="true">9.2.</strong> OpenStreetMap viewer</a></li><li class="chapter-item expanded "><a href="../side_projects/fifteen_min.html"><strong aria-hidden="true">9.3.</strong> 15-minute neighborhoods explorer</a></li></ol></li></ol>
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<h1 class="menu-title">A/B Street</h1>
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<main>
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<h1><a class="header" href="#map-model" id="map-model">Map model</a></h1>
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<p>A/B Street transforms OpenStreetMap (OSM) data into a detailed geometric and
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semantic representation of the world for traffic simulation. This chapter
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describes that map model, with the hopes that it'll be useful for purposes
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beyond this project.</p>
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<h2><a class="header" href="#overview" id="overview">Overview</a></h2>
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<p>A <code>Map</code> covers everything inside some hand-drawn boundary, usually scoped to a
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city or a few of a city's districts. Unlike OSM, it doesn't cover the entire
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world; it only has areas specifically extracted for some purpose.</p>
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<p>A map consists of many objects. Mainly, there are roads, broken down into
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individual lanes, and intersections. A road is a single segment connecting
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exactly two intersections (as opposed to OSM, where a single "way" may span many
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intersections). Lanes within a road have a specific type, which dictates their
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direction of travel (or lack of travel, like on-street parking) and uses.
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Sidewalks are represented as bidirectional lanes. Roads connect at
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intersections, which contain an explicit set of turns, each linking a source
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lane to a destination lane.</p>
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<p>Maps also contain parking lots and buildings, which connect to the nearest
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driveable lane and a sidewalk. Maps have water and park areas, only used for
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drawing. They also represent public transit stops and routes.</p>
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<h2><a class="header" href="#how-is-a-map-used" id="how-is-a-map-used">How is a map used?</a></h2>
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<p>Unlike some GIS systems, maps don't use any kind of database -- they're just a
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file, anywhere from 1 to ~500MB (depending on the size of their boundary). Once
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loaded into memory, different objects from the map can be accessed directly,
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along with a large API to perform various queries.</p>
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<p>Most of the map's API is read-only; once built, a map doesn't change until
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user-created edits are applied.</p>
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<p>The pipeline to import a map from OSM data (and also optional supplementary,
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city-specific data) is complex and may take a few minutes to run, but it happens
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once offline. Applications using maps just read the final file.</p>
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<h2><a class="header" href="#features" id="features">Features</a></h2>
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<p>Why use A/B Street's map model instead of processing OSM directly?</p>
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<p>TODO: Order these better. For each one, show before/after pictures</p>
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<h3><a class="header" href="#area-clipping" id="area-clipping">Area clipping</a></h3>
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<p>Bodies of water, forests, parks, and other areas are represented in OSM as
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relations, requiring the user to stitch together multiple polylines in undefined
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orders and handle inner holes. A/B Street maps handle all of that, and also clip
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the area's polygon to the boundary of the entire map -- including coastlines.</p>
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<h3><a class="header" href="#road-and-intersection-geometry" id="road-and-intersection-geometry">Road and intersection geometry</a></h3>
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<p>OSM represents roads as a polyline of the physical center of the road. A/B
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Street infers the number and type of lanes from OSM metadata, then creates
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individual lanes of appropriate width, each with a center-line and polygon for
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geometry. At intersections, the roads and lanes are "trimmed back" to avoid
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overlapping, and the "common area" becomes the intersection's polygon. This
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heuristic process is reasonably robust to complex shapes, with special treatment
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of highway on/off-ramps, although it does still have some bugs.</p>
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<h3><a class="header" href="#turns" id="turns">Turns</a></h3>
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<p>At each intersection, A/B Street infers all legal movements between vehicle
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lanes and sidewalks. This process makes use of OSM metadata about turn lanes,
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inferring reasonable defaults for multi-lane roads. OSM turn restriction
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relations, which may span a sequence of several roads to describe U-turns around
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complex intersections, are also used.</p>
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<h3><a class="header" href="#parking-lots" id="parking-lots">Parking lots</a></h3>
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<p>OSM models parking lots as areas along with the driveable aisles. Usually the
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capacity of a lot isn't tagged. A/B Street automatically fills paring lots with
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individual stalls along the aisles, estimating the capacity just from this
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geometry.</p>
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<h3><a class="header" href="#stop-signs" id="stop-signs">Stop signs</a></h3>
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<p>At unsignalized intersections, A/B Street infers which roads have to stop, and
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which have right-of-way.</p>
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<h3><a class="header" href="#traffic-signals" id="traffic-signals">Traffic signals</a></h3>
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<p>OSM has no way to describe how traffic signals are configured. A/B Street models
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fixed-timer signals, automatically inferring the number of phases, their
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duration, and the movements that are prioritized and permitted during each
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phase.</p>
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<h3><a class="header" href="#pathfinding" id="pathfinding">Pathfinding</a></h3>
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<p>A/B Street can determine routes along lanes and turns for vehicles and
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pedestrians. These routes obey OSM's turn restriction relations that span
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multiple road segments. They also avoid roads that're tagged as not allowing
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through-traffic, depending on the route's origin and destination and vehicle
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type. The pathfinding optionally makes use of contraction hierarchies to greatly
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speed up query performance, at the cost of a slower offline importing process.</p>
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<h3><a class="header" href="#bridge-z-ordering" id="bridge-z-ordering">Bridge z-ordering</a></h3>
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<p>OSM tags bridges and tunnels, but the roads that happen to pass underneath
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bridges aren't mapped. A/B Street detects these and represents the z-order for
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drawing.</p>
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<h3><a class="header" href="#buildings" id="buildings">Buildings</a></h3>
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<p>Similar to areas, A/B Street consolidates the geometry of OSM buildings, which
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may be split into multiple polygons. Each building is also associated with the
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nearest driveable lane and sidewalk, and metadata is used to infer a land-use
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(like residential and commercial) and commercial amenities available.</p>
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<h3><a class="header" href="#experimental-public-transit" id="experimental-public-transit">Experimental: public transit</a></h3>
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<p>A/B Street uses bus stops and route relations from OSM to build a model of
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public transit routes. OSM makes few guarantees about how the specifics of the
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route are specified, but A/B Street produces specific paths, handling clipping
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to the map boundary.</p>
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<p>... All of this isn't the case yet, but it's a WIP!</p>
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<h3><a class="header" href="#experimental-separated-cyclepaths-tramways-and-walking-paths" id="experimental-separated-cyclepaths-tramways-and-walking-paths">Experimental: separated cyclepaths, tramways, and walking paths</a></h3>
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<p>Some cyclepaths, tram lines, and footpaths in OSM are tagged as separate ways,
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with no association to a "main" road. Sometimes this is true -- they're
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independent trails that only occasionally cross roads. But often they run
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alongside a road. A/B Street attempts to detect these and "snap" them to the
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main road as extra lanes.</p>
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<p>... But this doesn't work yet at all.</p>
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<script src="../book.js" type="text/javascript" charset="utf-8"></script>
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</body>
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</html>
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