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Vijka

A demand model/simulator for highway tolls on regional road networks

About

Vijka is a simulator for estimating and analyzing the number of vehicles that will use a highway, that is, its traffic volume.

Given a road network and a weighted list of origin/destination trip desires, Vijka can perform the necessary demand assignments and output the traffic volume estimates and other interesting information.

Vijka is fast, and has been used to run, in only a couple of minutes, multiple scenarios (10+) of a large number of trips (each with its own cost perception) on very detailed networks (~100k nodes, ~200k links).

Interface and platforms

Vijka runs on many different platforms, both native and virtual. For the time being, it only has a console based user interface. You open it in a shell (terminal emulator or prompt) and interact with it via text commands, each with a set of corresponding arguments.

Of the virtual platforms, it runs on both the Neko VM and the Java VM (or, more precisely, the various Java VM flavors out there). Of these, the Java VM is recommended due to performance and scalability reasons.

Vijka also runs natively on all major operating systems (Windows, Mac OSX and Linux), being compiled through a C++ tool-chain. Not that anyone would need that, but it should also run on mobile systems (iOS, Android)...

Ideally, the native (C++ based) binaries should be faster than the Java VM (JVM) and, therefore, recommended for large networks. However, due mainly to garbage collector issues (memory management) on the underlying C++ toolchain, that is not true yet.

Therefore, the amazing Hotspot Java VM (be it the Sun/Oracle one, or the OpenJDK one for the Linux users) is still the fastest and, so, the recommended alternative. It should also be noted that the JVM is quite faster on Linux than on Windows.

Input

The various inputs for Vijka may be split in two categories: network and demand.

The network consists of a collection of nodes and links (roads connecting nodes).

Toll fares can be set on any link. Several vehicles can be defined, each with different number of axes and with different toll fare multipliers (the fare set on the link is only the base fare). Links also have types, and for each combination of link type and vehicle a different speed may be set.

Links can also have custom (more detailed) shapes, other than the default start node -> finish node line segment. They can also be referenced by aliases; aliases and links have N:M relation.

The demand consists on a weighted list of origin/destination trip desires. Each of its elements is associated with a particular vehicle type and can have a different perception of costs: there are distance based costs and time based costs (the latter ones are then divided into the social or operational categories).

All input is done in text files, following the Elebeta Text Table (ETT) format. This is a variation of Comma-Separated Values (CSV, RFC 4180) that retains backwards compatibility but includes additional information:

  • Separator: comma, tab, semicolon, pipe?
  • Encoding: local ISO/extended ASCII or UTF-8?
  • Mandatory column names
  • Mandatory column types (these are used by software to check that what's in a column is actually compatible with its definition).

Assignment

The assignment is of the all-or-nothing type, and is computed as the shortest path between origin and destination, according to a generalized cost function.

Output

As with other traffic assignment software, one can generate traffic volume estimates for all (used) links. However, this is not all.

Vijka was developed with a particular focus on analyzing users of a particular road segment. Therefore, for each assigned origin/destination trip desire a result is stored that contains all cost parts (and the resulting generalized cost) and its complete path. Since the full path is stored, Vijka can quickly and easily identify and analyze users (and non users) of a particular link.

Vijka outputs its results in the ETT format and, also, when applicable, in GeoJSON. GeoJSON is a subset of JSON (a popular text based object representation format) used for sharing geospacial information, that can be easily parsed, converted to other formats such as shapefiles (using ogr2ogr) and viewed (using Quantum GIS, TileMill, or even GitHub itself).

Other features

Vijka has some additional features...

For instance, you can select and edit a group of links in the (distance based) shortest path between two points. This sometimes saves you from having to manually select several hundred links.

There are also features for batch runs and corresponding storage of their results, but these are still somewhat primitive: you can save and execute a series of commands from a text file – but there is no support for variables or control statements yet – and you can also save and recall results for multiple (different) runs.

Implementation details

Vijka is entirely coded in Haxe, a open-source cross-platform programming language and toolkit under active development.

The Haxe programming language is a high-level procedural and object oriented language, with EcmaScript like syntax and that has also inhered a few improvements from functional languages. It is strictly typed (so it has a lot of compile time checks, unlike Python), but has generics (unlike C) and real runtime Dynamics (unlike C++). It is also less verbose, easier to read, write and maintain than languages like Java or C++. Of the functional like improvements, it supports function binding, function passing, Lambda calculus and pattern matching. Haxe is used to target both native (desktop C++, iOS, Android, ...) and virtual (Neko VM, Java VM, JavaScript, Flash, ...) targets, although some code refactoring is sometimes needed due to incompatibilities between the underlying platforms.

Dependencies

Vijka depends on Haxe 3 and on the following Haxe libraries:

  • elebeta-format, for ETT and CSV readers and writers
  • [mcli], for command processing
  • [hscript], for the query engine

[hscript]:https://github.com/HaxeFoundation/hscript) [mcli]:https://github.com/waneck/mcli

Development history

Vijka was created for a highway concession study, and version 1.0 was aimed to provide only the most important functionality.

Initially not a concern, performance ended up getting a lot of importance when GC issues were found on C++ Haxe target. In the next two days, the migration to the (at time thought to be yet experimental) Java target and several algorithm optimizations generated a 2000x speed improvement. A few days later, another 3~4x improvement was experienced by making the computation of paths concurrent.

Future plans

In addition to general improvements on functionality and performance, there are a few major changes that may get implemented in the near future:

  • a simple GUI interface
  • a self-contained network viewer and editor

Other than this, most of the code will eventually be unit tested and documented.

Creating a generic traffic simulator was never a goal for Vijka, but it may end up being its natural evolution.