Battery backup systems (BBS) are equivalent to a UPS that keeps traffic signals running for two or more hours after a power failure.   The photo shows a Type 332 controller cabinet (typical installation on a Caltrans highway) and the somewhat smaller cabinet on the right is the BBS.  BBS started seeing interest in California about 2001 when the state had an unusually high number of brownouts.   BBS started getting deployed in greater numbers in about 2004 after a CPUC study documented their benefits.  (See http://www.energy.ca.gov/reports/2004-05-05_400-04-006.PDF for the report.)   At that time the caveat was that they were not recommended until all the traffic signal lamps were converted from incandescent to LED, but for the last 2-3 years all new traffic signals have been installed with LED lamps.   The new Caltrans 2015 revised standard specification (published in January 2017) includes language that does not go so far as mandating BBS but does strongly infer that it is a good idea to include BBS in all new traffic signal installations.