abstreet/docs/how_it_works.md

How A/B Street works

The overview:

1. A detailed map of Seattle is built from OpenStreetMap (OSM)
2. A realistic set of daily trips by car, bike, foot, and bus are simulated
3. You make small changes to roads and intersections
4. You explore how these changes affect the trips

Details below. Many limitations are mentioned; improvements are ongoing. I'll add pictures to explain better when I get time.

• How A/B Street works
  • Driving
  • Parking
  • Biking
  • Walking
  • Transit
  • Intersections
  • People and trips
  • Map edits

Driving

• Movement: no acceleration, go the full speed limit of the road unless there's a slower vehicle in front
• Lanes
  • No over-taking or lane-changing in the middle of a road, only at intersections
  • Strange choice of lanes -- the least full at the time of arrival
  • Narrow two-way neighborhood roads where, in practice, only one car at a time can go are currently full two-way roads
• Routing is based on fastest time assuming no traffic
  • No rerouting if the driver encounters a traffic jam

Parking

• Types
  • On-street: parallel parking lanes from GeoData blockface dataset and manually mapped
  • Off-street: most buildings have at least a few parking spots in a driveway or carport
    • Currently experimenting in the downtown map: set the number of available spots based on number of cars seeded at midnight
  • Parking lots: the number of spots is inferred
• Restrictions
  • All spots are public except for the few spots associated with each building
  • No time restrictions or modeling of payment
• How cars park
  • Drivers won't look for parking until they first reach their destination building. Then they'll drive to the nearest open parking spot (magically knowing what spots are open, even if they're a few blocks away). If somebody else has taken the spot when they arrive, they'll try again.
  • Once a driver finds an open spot, they'll take 10-15 seconds to park. They block the road behind them in the meantime. There are no conflicts between pedestrians and cars when using a driveway. Cars won't make left turns into or out of driveways.
• Some parking along the boundary of the map is "blackholed", meaning it's impossible to actually reach it. Nobody will use these spots.

Biking

• Choice of lane
  • Multi-use trails like the Burke Gilman and separated cycle-tracks like the one along Broadway are currently missing
  • Cyclists won't use an empty parking lane
  • On roads without a bike lane, cyclists currently won't stick to the rightmost lane
  • No over-taking yet, so cars can get stuck behind a bike even if there's a passing lane
• Elevation change isn't factored into route choice or speed yet; pretend everybody has an e-bike
• Beginning or ending a cycling trip takes 30-45 seconds. Locking up at bike racks with limited capacity isn't modeled; in practice, it's always easy in Seattle to find a place to lock up.

Walking

• Not using sidewalk and crosswalk data from OSM yet
• No jay-walking, even on empty residential streets
• Pedestrians can't use roads without sidewalks at all
  • When a road only has a sidewalk on one side, driveways will cross the road
• Pedestrians can "ghost" through each other; crowds of people can grow to any size

Transit

• The modeling of buses is extremely simple and buggy; I'll work on this soon
• No light rail yet

Intersections

• Conflicting movements are coarse: a second vehicle won't start a conflicting turn, even if the first vehicle is physically out of the way but still partially in the intersection
• Most of the time, vehicles won't "block the box" -- if there's no room in the target lane, a vehicle won't start turning and risk getting stuck in the intersection
• Traffic signals
  • Only fixed timers; no actuated signals or centralized control yet
  • The timing and phases are automatically guessed, except some intersections are manually mapped
  • No pedestrian beg buttons; walk signals always come on
  • The signal doesn't change for rush hour or weekday/weekend traffic; there's one pattern all day
• Turn restrictions from OSM are applied
  • Per lane (left turn only from leftmost lane), entire roads, multiple intersections

People and trips

• A "synthetic population" of ~700,000 people come from PSRC's Soundcast model
  • Soundcast uses census, land-use, vehicle counts, and commuter surveys. The current data is from 2014.
  • All driving trips are currently single-occupancy; no car-pooling or ridesharing
  • Parked cars are initially placed at midnight based on the number of trips between buildings
• Each person's schedule never changes
  • Your changes to the map won't yet convince somebody to take a bus or walk instead of drive

Map edits

• Types of edits
  • Change types of lanes. Sometimes this is unrealistic based on actual road width, but data for this is unavailable.
  • Reversing direction of lanes
  • Changing stop signs
  • Changing traffic signal timing
  • Closing roads and intersections for construction, forcing rerouting
• Disconnecting the map
  • Generally you can't close sidewalks or make changes to make buildings unreachable
  • You shouldn't be able to make bus stops unreachable, but currently this is buggy