How to access route parameters for generating links?

[jasonmitchell]

Similar to the scenario in #42.

Assuming a route like /race/123/cars/456 and the following model for a car resource:

public class Car
{
    public int Id { get; set; }
    public string DriverName { get; set; }
    public string TeamName { get; set; }
}

How would do we support getting the race id 123 to generate links without needing to modify the model to support generating the state?