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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/map_parking.html"><strong aria-hidden="true">2.1.</strong> How to map on-street parking</a></li><li class="chapter-item expanded "><a href="../howto/new_city.html"><strong aria-hidden="true">2.2.</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></ol></li><li class="chapter-item expanded "><a href="../map/index.html"><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" class="active"><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></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="#gridlock" id="gridlock">Gridlock</a></h1>
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<p>Here "gridlock" refers to the general problem of trips getting permanently
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stuck, preventing the full simulation from completing. Most of the work is
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tracked <a href="https://github.com/dabreegster/abstreet/issues/114">here</a>.</p>
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<p>The general lesson is: you can't code your way around all edge cases. The data
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in OSM often needs manual fixes. It's often useful to spend coding effort on
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tools to detect and fix OSM problems.</p>
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<h2><a class="header" href="#problems" id="problems">Problems</a></h2>
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<p>The choices in the movement model matter. Some gridlock is inherent to any
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system with queueing and conflicting turns. But in reality, people wiggle around
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partly blocked turns. And some of this comes from the treatment of the
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front/back of vehicles.</p>
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<ul>
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<li>Short roads in OSM causing very weird geometry</li>
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<li>Intersection geometry being too large, requiring too much time to cross</li>
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<li>Unrealistic traffic patterns caused by everyone trying to park in one big
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garage (downtown) or take some alley (the UW soundcast issue)</li>
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<li>Too many people try to take an unprotected left turn (often at a stop sign)</li>
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<li>Bad individual traffic signals, usually at 5- or 6-ways</li>
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<li>Groups of traffic signals logically acting as a single intersection</li>
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<li>Separate traffic signals along a corridor being unsynchronized</li>
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<li>Vehicles performing illegal sequences of turns</li>
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<li>Vehicles are stuck with their plan and not reacting to traffic by changing
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route</li>
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<li>Real traffic would result in a gridlock without a deliberate actions to avoid
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it. Such actions range from individual decisions of drivers to police manually
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controlling traffic. Intelligent avoidance of gridlock is not simulated and is
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extremely hard to simulate.</li>
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<li>Vehicles will wait in lane filled with already waiting vehicles, even if there
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is a completely empty lane allowing travel in desired direction. It makes
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easier for entire lane between crossings to fill, contributing to gridlocks.
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Note that while this and other clearly stupid behaviors are clearly
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unrealistic, it is not trivial to implement more realistic and more efficient
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decisions.</li>
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<li>Issues caused by the unrealistic
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<a href="discrete_event.html#lane-changing">lane-changing model</a>
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<ul>
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<li>Two turns that go to the same lane (one going "straight", the other often a
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lane-change) conflict. The conflict is coarse, at the granularity of the
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entire intersection. So if vehicles are piled up in two lanes trying to
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merge into one, then one group is likely to go through as expected, but the
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second group will wait for the first to completely clear the intersection.
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Until then, it looks like a conflicting turn is being done.</li>
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</ul>
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</li>
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</ul>
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<h2><a class="header" href="#solutions" id="solutions">Solutions</a></h2>
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<p>Divide into implemented or not.</p>
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<ul>
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<li>Synchronizing pairs of signals</li>
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<li>Uber-turns
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<ul>
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<li>for interpreting OSM turn restrictions</li>
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<li>for synchronizing a group of signals</li>
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<li>for locking turn sequences
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<ul>
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<li>Once a vehicle starts an uber-turn, prevent others from starting
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conflicting turns on nearby intersections. Until groups of traffic signals
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are configured as one, this is necessary to prevent somebody from making
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it halfway through a sequence then getting blocked.</li>
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</ul>
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</li>
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</ul>
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</li>
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<li>Cycle detector</li>
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<li>block-the-box protection
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<ul>
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<li>the manual list of overrides</li>
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<li>likely shouldn't apply during uber-turns</li>
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<li>is it always fine to block the box at degenerate intersections?</li>
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</ul>
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</li>
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<li>hacks to allow conflicting turns at really broken intersections</li>
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<li>manually timing signals</li>
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<li>penalties for lane choice to make lane usage realistic</li>
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</ul>
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<h3><a class="header" href="#not-implemented" id="not-implemented">Not implemented</a></h3>
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<ul>
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<li>Dynamic rerouting</li>
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<li>Allow multiple vehicles through intersection at once if there is enough space
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on lane where given vehicle is going. Currrently vehicles travel through
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crossings one by one (or, with <code>--disable_block_the_box</code> enabled - will enter
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crossing even if leaving it will be impossible).</li>
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<li>Last resort: if someone's waiting on a turn >5m, just go.</li>
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<li>Uber-turns
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<ul>
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<li>Group both stop sign and traffic signal intersections when looking for
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uber-turns. Even a single traffic signal surrounded by tiny roads with stop
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signs is causing problems.</li>
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</ul>
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</li>
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</ul>
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<h2><a class="header" href="#strategy-for-resolving" id="strategy-for-resolving">Strategy for resolving</a></h2>
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<p>Because there are so many different causes all tangled together, my approach is
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to simplify the simulation as much as possible. A problem is much easier to
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understand and fix when it's isolated. I've been trying this to get the downtown
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weekday scenario to complete. A list of different techniques to simplify, in no
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particular order:</p>
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<ul>
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<li>Use the <code>--infinite_parking</code> flag to just let everyone park directly in their
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destination buildings. This is useful since downtown has many large parking
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garages with high capacity, but I don't have a data source describing them.</li>
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<li>Use the <code>--disable_turn_conflicts</code> flag, which greatly reduces realism, but
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lets conflicting turns happen simultaneously. (Even with this and other flags,
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downtown still gridlocks!) It also disables traffic signals, so bad inferred
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timing isn't an issue.</li>
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<li>Use the <code>--disable_block_the_box</code> flag to workaround short roads.</li>
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<li>If you notice problems forming from cars stacking up behind slower cyclists,
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there's no over-taking implemented yet. Use the scenario modifiers to convert
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all biking trip to driving:
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<code>--scenario_modifiers='[{"ChangeMode":{"to_mode":"Drive","pct_ppl":100,"departure_filter":[0.0,86400.0],"from_modes":["Bike"]}}]'</code></li>
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<li>If all else fails, use the scenario modifiers to bluntly cancel some
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percentage of all trips.</li>
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</ul>
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<h2><a class="header" href="#fixing-data-used-in-simulation" id="fixing-data-used-in-simulation">Fixing data used in simulation</a></h2>
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<p>Give more examples of changesets.</p>
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<ul>
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<li>upstreaming turn restrictions into OSM to prevent invalid U-turns and other
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crazy movements
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<ul>
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<li>ex: <a href="https://www.openstreetmap.org/changeset/87945050">https://www.openstreetmap.org/changeset/87945050</a></li>
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</ul>
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</li>
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<li>upstreaming lane count fixes into OSM to improve geometry</li>
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</ul>
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</body>
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</html>
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