Hi, the support of alignment is new to IFC, first introduced in IFC4.1 and updated in IFC4.2 (currently candidate status). Check technical website for definitions.
In IFC4.1, only the IfcAlignment can hold property sets, where you can add such information for the whole alignment.
In IFC4.2 there is a new entity, IfcReferent, that defines a certain position along the alignment from which onwards certain criteria are applied. You can use this to define a design speed and other criteria.
IFC4.3 (in development) will add additional support for road and railway, this is what @sergej refers to.