The blasts, which caused no injuries, came a day after Spanish politicians expressed hope that the Irish Republican Army's announcement that it was laying down arms might encourage ETA to do likewise.
Police had been advised where and when the bombs would explode after a warning call in the name of ETA to the pro-Basque independence newspaper Gara, said an Interior Ministry statement. ETA regularly warns of its attacks through calls to Gara.
The first blast occurred at 6pm about 46 miles southwest of Madrid on the A5 highway. A second device exploded 87 miles south of Madrid along the A4 highway 15 minutes later.
The ministry said both devices exploded on the shoulder of the roads causing little damage.
The blasts happened as millions of Spaniards took to the roads to begin their summer holidays. Traffic on both highways was interrupted by the blasts but was allowed resume about two hours later.
"ETA's aim this afternoon was to upset the thousands of citizens who were setting about enjoying their holidays," the ministry statement said.
The blasts came hours after authorities announced that French police had arrested four suspected ETA members in raids Thursday.
Also yesterday, two former leaders of the group were sentenced to 32 years in prison for ordering the kidnapping of a prison official in 1996.