9.4 Spiral

A spiral is a clothoid that describes a changing curvature of the road reference line.

Figure 27. Road geometry described by a spiral

Figure 27 shows the concept of a spiral. Spirals may be used to describe the transition from a <line> element to an <arc> element without causing leaps in the curvature.

A spiral is characterized by the curvature at its start position (@curvStart) and the curvature at its end position (@curvEnd). Along the arc length of the spiral (see @length of the <geometry> element), the curvature is linear from the start to the end.

It is also possible to arrange several <line>, <spiral>, and <arc> elements in a sequence in order to describe complex curvatures.

Elements in UML model

<spiral> element

In ASAM OpenDRIVE, a spiral is represented by the <spiral> element within the <geometry> element.

UML class: t_road_planView_geometry_spiral
XML tag:   <spiral>

Spirals or more specifically Euler spirals also known as clothoids. They describe road reference lines with constantly changing curvatures.

Table 19. Attributes of the <spiral> element
Name	Type	Use	Unit	Description
`curvEnd`	double	required	1/m	Curvature at the end of the element. Positive curvature: left curve (counter-clockwise motion) Negative curvature: right curve (clockwise motion)
`curvStart`	double	required	1/m	Curvature at the start of the element. Positive curvature: left curve (counter-clockwise motion) Negative curvature: right curve (clockwise motion)

XML example

<geometry s="100.0" x="38.00" y="-1.81" hdg="0.33" length="30.00">
    <spiral curvStart="0.0" curvEnd="0.013"/>
</geometry>

Rules

The following rules apply to spirals:

@curvStart and @curvEnd should not be the same.

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