The discussion on the longitudinal gap between vehicles is well-established; increased speed means increased gap. But this only covers the gap in the direction of travel. What information is there on the lateral gap – the one between vehicles at the same level travelling in adjoining traffic lanes.
From German practice
Practice in Germany sets up the idea of vehicle envelopes, and adds an amount for the space between them. The total width of these clearance spaces and vehicle envelopes gives the overall clearance envelope. The following example from (ref. 1617) shows the idea, with
- “vehicle envelope” (Verkehrsraum ) (the dotted line in the figure)
- “clearance envelope” (Lichter Raum) (the solid line in the figure)
You will see from the figure that
- There is a 750 mm clearance between the bicycle envelope and the truck envelope
- The vehicle envelope for the truck includes width “as used” – that is, there is an additional width for wing mirrors
- The width of roadway for a bicycle lane and a truck lane – that is, the overall clearance envelope in this example is 5.70m, so that
- For a 3.50m traffic lane the cycle lane width should be 2.20m (and a 1.50m cycle lane would not be adequate)
From multi-country (Austroads) practice
Austroads suggests an additional aspect to the lateral gap between vehicles: that there can be an inter-action between them. On inter-action between trucks and bicycle riders, (ref. 1887) says that
“Due to the side ‘wind’ force exerted on bicycle riders from heavy vehicles, roads should be designed to provide satisfactory clearances between the bicycle envelope and the vehicle. Table 4.16 lists the desirable clearances that should be provided to enhance cyclist safety. …. Similar clearances to cars should be provided in order that cyclists do not feel unduly threatened by general motor traffic.”
(Table from ref. 1887)
Two points from this table are:
1. For a cyclist envelope of 1.0m then, even at a 60 km/hr speed limit, the cycle lane width should be at least 2.0m.
2. The suggested clearance increases with speed
The details given from German practice indicate a modular approach to cross-section design. This approach could be applied to several other aspects of highway geometric design, and would introduce a revolution in the field, not least in terms of greatly reducing the number of design standards which presently exist (and about time, too).
1617 – Germany, Wolf et at “Strassenplanung”, Werner Verlag 2013
1887 – multi-country, “AGRD part 3: Geometric design”, Austroads 2010