How the simulation works

Traffic models a five-kilometer stretch of two-lane highway. Every vehicle has a preferred speed, a physical length and a driver type. Vehicles accelerate towards their preferred speed, slow for traffic ahead and change lanes when enough room is available.

Reading the road

Traffic moves from left to right. At the end of a stripe, the road continues on the stripe immediately below it.

Running the simulation

Start: Begins a new run. After a run is stopped or loaded, this button becomes Resume and continues that run. While running, all action buttons except Stop are disabled.
Stop: Pauses the run without discarding its vehicles or statistics.
Clear: Removes all vehicles and lane closures, resets the clock and statistics, and returns the simulator to its initial state.
Save: Saves the current run to a file on your computer. It is available while the simulation is stopped. The file is created locally in your browser; no simulation data is sent to the web server.
Load: Restores a saved file from your computer, including its settings, vehicles, statistics and lane closures, in a stopped state. The file is read locally in your browser and is not uploaded to the web server.

Traffic flow

Time between vehicles: Sets the average arrival interval.
Arrival variability: Moves individual arrivals earlier or later around the average interval. The simulator always leaves at least 0.2 seconds between scheduled arrivals.
Truck / car ratio: Sets the relative number of trucks. A value of 1 / 10 produces approximately one truck for every ten cars over a sufficiently long run.

Driver behavior

Speed variability: Sets how much individual preferred speeds vary around the characteristic speed for each driver type. Conformist drivers prefer approximately 100 km/h, slow drivers 70 km/h and speeders 130 km/h.
Speeders: Sets the likelihood that a new vehicle is driven by a speeder. Together, Speeders and Slow drivers cannot exceed 100 percent.
Slow drivers: Sets the likelihood that a new vehicle is driven by a slow driver. Vehicles assigned neither type are conformist.
Trucks may pass: Allows trucks to move into the left lane to overtake. Turning it off does not prevent cars from passing trucks.
Passing on the right: Allows vehicles to overtake in either lane. When it is off, vehicles normally overtake on the left and return to the right lane when a safe opening appears.
Limit truck speed: Gives trucks a separate speed limit and reveals the two controls below it.
Truck limit: Sets the preferred speed limit for trucks.
Limit variability: Sets the speed variation between limited trucks.

Events and display

Left lane: Adds or removes a closure in the left lane just before the midpoint of the road. The switch is temporarily disabled while a vehicle is at or very close to the closure location.
Right lane: Adds or removes a closure in the right lane at the same road position. The switch has the same safety lockout as the left-lane closure.
Simulation speed: Sets how quickly simulated time passes relative to real time. For example, 2× attempts to run two simulated seconds per real second. Higher settings demand more work from the browser and may be limited by the device.
Repeatable random seed: Starts each new run with the same pseudorandom sequence. It reproduces the same traffic when the other settings and measured display refresh rate are unchanged.
Stop after a fixed time: Ends the run automatically after the entered duration and reveals the two duration fields below it.
Minutes: Sets the whole-minute part of the fixed duration, from 0 to 120.
Seconds: Sets the additional seconds of the fixed duration, from 0 to 59.
Metric: Displays road lengths and speeds in kilometers, meters and km/h.
Imperial: Displays road lengths and speeds in miles, yards and mph. Changing units does not change the simulation itself.

Control settings and the selected units are retained in this browser for the next visit. Changes to controls take effect immediately.

Vehicle behavior

A vehicle considers changing lanes when it catches slower traffic. It checks the space ahead and behind before moving. Unless passing on the right is enabled, vehicles return to the right lane when a safe opening appears.

The browser edition intentionally follows the original Windows rules. Those rules can occasionally let vehicles overlap; as in the Windows app, they continue moving and separate again.

Timing

The model measures the display refresh rate as an integer and advances once per display update, matching the Windows timing model. A lane change takes 48 updates. The simulator also uses the Windows app's pseudorandom sequence, so a repeatable seed reproduces the same traffic when the settings and refresh rate are unchanged.

Privacy

Simulation data stays in your browser, and saved files remain on your computer. Neither are uploaded to the web server.