c# thread safety when referencing static properties on other classes

I have a method to generate fully qualified URLs that I wrote which I would like to have as static so its easy to call from models as needed.

I'm still having problems however with being able to decide if its thread safe or not.

Here is the code.

    public string GenerateURLFromModel(string action, string controller)
    {
        HttpContextWrapper wrapper = new HttpContextWrapper(HttpContext.Current);
        Uri url = HttpContext.Current.Request.Url;
        UrlHelper urlHelper = new UrlHelper(new RequestContext(wrapper, RouteTable.Routes.GetRouteData(wrapper)));

        return url.AbsoluteUri.Replace(url.PathAndQuery, urlHelper.Action(action, controller));
    }

What I know already is:

1) The two strings passed in will be thread safe since they are immutable reference types.

2) All objects instantiated within a static method can be considered thread safe since they exist only on the stack for that specific thread.

What I'm unsure of is:

1) How does the use of HttpContext.Current and RouteTable.Routes play in this method? They are both static properties that I'm passing into the constructors.

My questions are:

1) What are the implications of using these static properties?

2) Does the rest of my understanding of the safeness of this method ring true?

3) What rules can I keep in mind in the future to help determine thread safeness in situations like this?