Aliased route redirect

Noob question:

I have in my routes.db:

map.signup 'signup/:plan_name', :controller => 'accounts', :action => 'new'

No problem, but this page contains a form which when invalid does a render :action => 'new' which causes the browser URL to jump back to / Accounts

All works but looks ugly, how can I correct the render action to take me back to my aliased action (ie. /Signup/xxxx) ?

Many thanks all!