These are goat head thorns:
Whether you call it goat head, stickers, puncture vine, or sand bur, this plant is about the most obnoxious weed on the planet. It produces tiny goat head shaped burs that poke the foot and flatten bicycle tires. A large patch of them can even flatten the tires on a small car.
Puncture Vine is an invasive species of plant that originally came from Europe. The botanical name is Tribulus terrestris. This fast growing annual sends out a low forming, dense mat of tiny leaflets. The stems can grow up to 6 feet in length, covering large areas in a matter of a couple of months. They bloom in the early summer, producing tiny 5 petaled yellow flowers. The fruit of the puncture vine looks like a spiny 5 sided Maltese cross. After the individual fruit bursts open, it releases the goat head thorns.
The bike paths along the east side of 101 and west side of Vulcan between Encinitas and Leucadia Boulevards are absolutely infested with them. The photo above was after a very short jog along the path.
Goat heads will go straight through bike tires and give you multiple punctures. Tony Kranz hit them a couple weeks ago, and more recently an entire family's tires were massacred out on the same paths.
The city needs to remove this invasive, non-native species ASAP.