Gridlock

Here "gridlock" refers to the general problem of trips getting permanently stuck, preventing the full simulation from completing. Most of the work is tracked here.

My general approach right now to get a map working is to cancel some percent of all trips, look for individual problems, fix them, and repeat. Once the full day works, cancel less trips. It's easier to isolate the root cause of problems when there are less of them erupting simultaneously.

The general lesson is: you can't code your way around all edge cases. The data in OSM often needs manual fixes. It's often useful to spend coding effort on tools to detect and fix OSM problems.

Problems

The choices in the movement model matter. Some gridlock is inherent to any system with queueing and conflicting turns. But in reality, people wiggle around partly blocked turns. And some of this comes from the treatment of the front/back of vehicles.

  • Short roads in OSM causing very weird geometry
  • Intersection geometry being too large, requiring too much time to cross
  • Unrealistic traffic patterns caused by everyone trying to park in one big garage (downtown) or take some alley (the UW soundcast issue)
  • Too many people try to take an unprotected left turn (often at a stop sign)
  • Bad individual traffic signals, usually at 5- or 6-ways
  • Groups of traffic signals logically acting as a single intersection
  • Separate traffic signals along a corridor being unsynchronized
  • Vehicles performing illegal sequences of turns
  • Vehicles are stuck with their plan and not reacting to traffic by changing route
  • Real traffic would result in a gridlock without a deliberate actions to avoid it. Such actions range from individual decisions of drivers to police manually controlling traffic. Intelligent avoidance of gridlock is not simulated and is extremely hard to simulate.
  • Vehicles will wait in lane filled with already waiting vehicles, even if there is a completely empty lane allowing travel in desired direction. It makes easier for entire lane between crossings to fill, contributing to gridlocks. Note that while this and other clearly stupid behaviors are clearly unrealistic, it is not trivial to implement more realistic and more efficient decisions.
  • Issues caused by the unrealistic lane-changing model
    • Two turns that go to the same lane (one going "straight", the other often a lane-change) conflict. The conflict is coarse, at the granularity of the entire intersection. So if vehicles are piled up in two lanes trying to merge into one, then one group is likely to go through as expected, but the second group will wait for the first to completely clear the intersection. Until then, it looks like a conflicting turn is being done.

Solutions

Divide into implemented or not.

  • Synchronizing pairs of signals
  • Uber-turns
    • for interpreting OSM turn restrictions
    • for synchronizing a group of signals
    • for locking turn sequences
      • Once a vehicle starts an uber-turn, prevent others from starting conflicting turns on nearby intersections. Until groups of traffic signals are configured as one, this is necessary to prevent somebody from making it halfway through a sequence then getting blocked.
  • Cycle detector
  • block-the-box protection
    • the manual list of overrides
    • likely shouldn't apply during uber-turns
    • is it always fine to block the box at degenerate intersections?
  • hacks to allow conflicting turns at really broken intersections
  • manually timing signals
  • penalties for lane choice to make lane usage realistic

Not implemented

  • Dynamic rerouting
  • Allow multiple vehicles through intersection at once if there is enough space on lane where given vehicle is going. Currrently vehicles travel through crossings one by one (or, with --disable_block_the_box enabled - will enter crossing even if leaving it will be impossible).
  • Last resort: if someone's waiting on a turn >5m, just go.
  • Uber-turns
    • Group both stop sign and traffic signal intersections when looking for uber-turns. Even a single traffic signal surrounded by tiny roads with stop signs is causing problems.

Fixing data used in simulation

Give more examples of changesets.

  • upstreaming turn restrictions into OSM to prevent invalid U-turns and other crazy movements
    • ex: https://www.openstreetmap.org/changeset/87945050
  • upstreaming lane count fixes into OSM to improve geometry