The Greater Eastern is still under Preliminary & Detailed Engineering Design by Norken International, see link
http://www.norken.co.ke/projects_design.php
I have meet them on the ground poking small white sticks on the ground from the big turn on the Eastern bypass going in towards mwalimu farm along Gacheha farm. What I got from them is that there is a lot to evaluate before it is finally decided where the actual road lies.
General direction,It starts at Lukenya and exits just before Kilimabogo along Garissa road. There will be an intersection from the Eastern bypass with a 14km extention road joining the Greater Eastern were Ruiru river meets Athi river at mwalimu farm.