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A/B Street Features

Ever been on a bus stuck in traffic, wondering why there are cars parked on the road instead of a bus lane? This article overviews the features of A/B Street, an in-progress traffic simulation game set in Seattle. Players explore how small changes to road layout and intersection rules affect multi-modal trips of pedestrians, drivers, transit users, and cyclists. The game's mission is to make it fun and simple for anybody to test an idea to improve Seattle's traffic flow and, if the idea works well, to communicate it to others.

Core gameplay

Explore Seattle and observe how traffic currently flows.

After finding a problem, edit the map in a few ways:

  • change lane types (example: replace on-street parking with a bus-only lane)
  • change which roads stop or yield at a stop sign
  • change the phases and timing of a traffic signal (example: ban a left turn or give it a dedicated phase)

These are changes that could be prototyped in real life relatively cheaply. A/B Street's purpose is to explore improvements to Seattle that we could try tomorrow, not longer-term improvements like light rail extensions.

After making edits, you can see how the same traffic patterns behave. I'm currently working on a way to easily visualize and compare results with and without edits. The game is currently more of a sandbox, but these phases of exploring, editing, and evaluating will be tied together in a more game-friendly format.


A/B Street generates a detailed map of Seattle from OpenStreetMap (OSM), King County GIS, and a few other sources. It takes lots of processing to make a map suitable for simulating traffic and that's visually appealing for a game. This section describes some of these problems and solutions.

The portion of the code-base to transform and clean up the map are separate from the traffic simulation. If you see another use for this map, contact me and we'll figure out a format to export the data for your purposes. The code isn't Seattle-specific; most things work if you only feed in OpenStreetMap data, and plugging in another city's custom GIS data is probably not hard.


OSM models entire roads (crossing many intersections) coarsely, sometimes with some metadata about lane restrictions.

A/B Street breaks roads down into indidual lanes, automatically finding the geometry from the OSM road's center-line. Lane types and the number of lanes come from heuristics on the OSM metadata and from extra King County GIS shapefiles. Types of lanes include:

  • Regular driving lanes, usable by any vehicle
  • Sidewalks for pedestrian movement, including bus stops and paths to buildings
  • Bus- and bike-only lanes
  • On-street parking lanes, with individual parking spots

Intersections (geometry)

OSM doesn't explicitly model intersections at all; some roads just share points.

In A/B Street, lanes and intersections have disjoint geometry.

This means that cars and pedestrians stop and queue at the correct position before crossing an intersection.

The intersection geometry is calculated automatically, even for strangely-shaped cases.

OSM ways often have many "intersections" very close together. These appear as extremely short roads in A/B Street, which complicates traffic modeling.

These can be merged automatically, which works reasonably well sometimes.

But some cases are very complex; this is Montlake and 520 without merging short roads:

Montlake and 520 with merging doesn't look much better, so currently short road merging is still disabled.

Some highway on-ramps in OSM are modeled with particularly unusual geometry, overlapping an arterial road.

A/B Street detects and fixes these cases.

Intersections (semantics)

A/B Street models each turn through intersections, connecting an incoming lane to an outgoing lane. Some of these turns conflict, so cars can't perform them simultaneously. Currently stop signs and traffic signals are modeled (roundabouts act like all-way stops).

For stop-sign controlled intersections, the bigger road by default has priority.

Intersections controlled by traffic signals have a default set of timed phases. Players can edit these.


How should the boundary of the map be handled? Without proper clipping, roads and lakes go out-of-bounds, often with very strange, long roads to nowhere.

Proper clipping trims polygons to fit properly. Roads that cross the boundary terminate at special border intersections, which can model traffic flowing into or out of the map.


Light orange buildings are classified as residential, and dark orange as commercial. Additional data from King County GIS reveals how many units some apartments have. This could be used to generate a realistic number of trips between residential and commercial areas.

Traffic simulation

A/B Street simulates the movement of individual agents:

  • Cars move along lanes in a queue, only changing lanes at intersections.
  • Buses are cars that cycle between bus stops, waiting at each one to unload and load passengers.
  • Bikes are cars with a maximum speed limit.
  • Pedestrians move bidirectionally along sidewalks. They can pass through each other; they don't queue or otherwise wait except at intersections. (Seattle has very few places where pedestrian movement is significantly bottlenecked due to other pedestrians.)


A/B Street originally used a discrete timestep model, updating every agent every 0.1 seconds. This was unnecessarily complex and too slow, so it now uses a discrete-event simulation, only updating agents when an interesting transition happens. So while a car crosses a long lane, its exact position is only interpolated for drawing, and a full update happens when the car reaches the intersection or the end of the queued cars.

On my modest laptop (7th-gen Intel i5, 8GB RAM), I can simulate 10,000 agents on the small map at 2x speed (one minute of game-time in 30 seconds). This definitely needs improvement, but many interesting scenarios will be around this scale. Simulating ~800,000 agents in all of Seattle is not a high priority; the flow into and out of a smaller region can be modeled much more cheaply.

A/B Street has a time-travel mode, useful for rewinding to trace the source of a bottleneck. This is a bit memory-intensive right now, but hasn't been updated to take advantage of the discrete-event model.


Most trips are multi-modal. A pedestrian will appear at a building, travel down the front path to the sidewalk, walk to a parked car they own or a bus stop, start driving or riding, park or deboard, and walk to their final destination. Bicycle trips have a fixed time to start or stop; this models how easy it is to find bike parking in Seattle. In contrast, car parking is often scarce, so drivers sometimes reach their destination, but start roaming around until they find an available spot.

Trip generation -- travel from here to there, starting at this time, using a car/bike/bus -- is currently very unrealistic. Players can manually define groups of cars to spawn uniformly from somewhere within a region they outline, but this is tedious. I'm actively looking for data sources (like the U.S census) that'll give hints about where people live and work, to generate more realistic demand data.

A/B Tests

Traffic simulation is fully deterministic -- run exactly the same scenario twice with the same version of the game, and you'll get the same results. Map edits like adding or removing parking will change the initial conditions minimally (so parked cars on unedited roads will usually not change). This lets players run meaningful A/B tests, holding everything fixed except for a few tweaks to lanes and intersections. Two simulations can be run in parallel, and there are tools to visualize how individual agents are taking different paths or moving faster/slower between the two runs.

Ongoing work

A/B Street is not yet generally playable (but if you want to anyway...):

  • The user interface to explore and edit the map is quite clunky.
  • The pieces of the game -- editing the map, running a simulation, comparing results -- exist, but nothing is tied together yet in a game-like format.
  • Data sources describing a realistic set of trips is missing; cars start and end at uniformly chosen places.
  • Some important things aren't yet modeled: light rail, big bike trails like the Burke Gilman, ridesharing services, off-street parking lots and garages.

If you're interested in joining me and working on problems like these, please get in touch. Funding is available. I also have half-finished articles with technical details about how A/B Street works; just ask me to finish them. Contact Dustin Carlino at

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