Multidrop one or two lane bus
Most of the suggestions so far  are based on use of RS422 differential signalling as per my original comment, used  in various ways and/or protocols that build on top of it. If you want to "roll your own" you'll find RS422 an excellent place to start looking. Hardware is available and well priced and performance under various loadings and distances is well described in the literature. 

RS422  - [Pandoras box]( all link to web  pages)
RS422 multidrop as above

QUICK REFERENCE FOR RS485, RS422, RS232 AND RS423 

Wikipedia RS422

Maxim tutorial 723Selecting and Using RS-232, RS-422, and RS-485 Serial Data Standards

TI RS422 products page - very valuable - even if you don't use their ICs ultimately 

Cross Reference - Application Notes - Training - Tools &amp; Software - Block Diagrams
Analog eLAB ™

They say


Texas Instruments is the world leader in RS-422, with a selection of transceivers for any application. TI provides industry-standard RS-422 solutions for industrial automation, motion control, e-meters, security electronics, building automation and hundreds of other applications where robust communication with high noise-immunity is needed over long cable lengths. TI offers RS-422 with multiple drivers/receivers in a single package and multiple temperature ranges.


RS485.COM -  Commercial page but VERY useful

Their FAQ

RS485 links



NatSemi RS485 application notes

NatSemiRS485 overview

Other:

Basic 422 intro