Junction modelling
Drawing paths and nodes
Effective journey planning begins with an accurate map of the highway network.
Maps usually record the centre-lines of roads and where they join. For instance, a junction between a busy main road and a side road might be drawn as shown on this photograph:
The junction of the centre-lines of Lensfield Road (main road, blue), and Tennis Court Road (quiet street, black).
The centre-lines of roads are drawn as a connected series of points that form a path. Where two paths join at the same point this is known as a node. Journey planner can only find a route from one path to another if they are joined at a node.
Showing a node (green dot) at the junction of the three paths.
So, the first principle in drawing maps for the journey planner is:
When there is a route from one path to another, it is essential that the two paths are joined at a node.
Detailed Junctions
Using the centre-lines of roads for journey planning has limitations, as is evident in the first photograph above. The cyclist is in a right turn traffic lane which is not represented in the map of the two connecting lines. The cyclist is going to turn into a one-way street with a contra-flow cycle lane. Better journey planning can be accomplished by capturing all those details.
Showing main traffic paths at this junction.
This photo shows all the main traffic paths at this junction along each lane of the carriageway. When drawn like this the roads do not join at a single node. Each maneouvre through the junction needs to be drawn.
Detail showing all the paths and maneouvres through the junction. The pink lines are maneouvres of type "Cross busy road".
In this picture all the paths and maneouvres that cyclists can perform are represented. All of the paths shown are one way, but the arrows are ommitted to avoid cluttering the image. Where paths join, a node is shown as a filled colour circle. The colour of the circle indicates the number of paths that join at that node. Yellow means 2, and green means 3. (This colour scheme comes from the game of snooker in which balls have points from one to seven according to their colour: red, yellow, green, brown, blue, pink, black.)
The busy paths are shown using a blue line, and they correspond to the provision type "Busy Road". The red line is the provision type "Cycle Lane", and the black is "Quiet Street". The maneouvres are also paths, but are shorter and more curved and they correspond to the turns. The turns into and out of Tennis Court Road are drawn in a pink colour, that corresponds to the provision type "Cross busy road".
To see how this junction appears as drawn on the map, view the junction of Lensfield Road with Tennis Court Road on the Edit network page.
Although it might look relatively complicated at first sight, this network captures a lot of information about the junction. For instance, the "Cross busy road" provision type acknowledges this is an advanced maneouvre, and that there is quite often a delay before the rider is able to make it. That information is used by journey planner when searching for either quietest or fastest routes through this junction. It might conclude that an alternative route, that turns right further down the road is faster, or that a route that uses a nearby crossing is quieter.
Drawing all the paths through a junction and ensuring that they are all properly joined is a time-consuming activity. However because the journey planner can handle both basic and detailed junction descriptions it can be treated as an incremental task. Start by drawing basic routing maps of your area. This enables the journey planner to do basic route planning. Sometime later the map can be edited with full path details for as few or as many junctions as you please.
Ways
In the first photograph it was easy to name the paths. The blue path was Lensfield Road, and the black path was Tennis Court Road. Now that all the paths through the junction are drawn there needs to be a consistent way of labelling all the paths.
Paths are grouped together into an object called a way. All the paths on a road usually belong to the same way. So here, there will be one way called Lensfield Road, and one called Tennis Court Road.
In the last picture of the junction there are three blue lines that represent the paths along Lensfield Road. They each belong to the Lensfield Road way. Similarly the black and red paths belong to the Tennis Court Road way.
That leaves the maneouvres. These short paths have ends in both ways. So the convention adopted is that a maneouvre belongs to the same way that it starts from. For instance the path that shows the left turn into Tennis Court road from Lensfield Road belongs to the Lensfield Road way, but the right turn out of the cycle lane belongs to the Tennis Court Road way.
