HOW TO DRAW ROUNDABOUTS
This article is on how to correctly draw roundabouts for Garmin auto routing GPS maps ©
When the correct roundabout polylines are used in Garmin maps, the GPS unit will give correct instructions such as
"...Enter roundabout and take second exit to Queen Victoria Drive..."
- Remember key point is to draw polyline in correct direction of traffic flow !
- Draw clockwise direction for traffic in UK, Thailand, Malaysia etc or
- Counter clockwise direction for traffic in USA, Laos, Vietnam etc
- After drawing polyline, make sure that one way box is ticked on both Properties and Routing tab.
- Select "Roundabout" in Properties tab and also under Routing tab (which is actually called Collector/Roundabout)
- Must draw this in two sections - ie must not be a single circular polyline as auto routing does not allow polyline to self intersect!
- Remember to connect nodes when finished.
- When drawing in roundabouts - need to be careful not to place too many nodes closer than 5.4m otherwise it may cause problems in compiling - easy method to overcome this is to make the roundabout slightly larger.
- One extra node between two intersections is usually enough for small to medium sized roundabouts (eg. total of 8 nodes in a roundabout with 4 streets should suffice.
- If drawing in some one way lanes with triangular dividers, I use 12 nodes to maintain a circular shape.
- I have found out that including too many nodes on the roundabout causes the compilation process to arbitrarily generalise the nodes and actually sometimes <>severely distorting the shape of the roundabout!)
- I also set all my roundabouts at 20kph speed limit (could use 40kph at larger ones) as realistically most of the time this is about the speed of traffic going round a busy roundabout. I am trying to aiming for auto routing on the maps to be able to estimate a more accurate time to destination.
- Here is an example of a roundabout template I use...
- Roundabout (clockwise) drawn for left hand side traffic flow
- eg. UK, Malaysia, Australia, NZ etc
- NOT USA, Europe, Vietnam, Laos etc
- Ignore labeling of roundabout, not necessary to do so.
- Click diagram for larger display
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