Handicap placards are issued by each state's department of motor vehicles and, once issued, are valid in most states. California requires visitors to obtain a travel placard, which is a temporary placard to use in the state while you're there. Parking without a placard can result in fines.
A handicap parking placard is placed on the rearview mirror of a vehicle, and allows for parking in specially designated spaces. The placard must be visible to park in the space, forgetting to hang it from the rearview mirror is not an excuse. It is issued by the state that the person who requires the privilege lives in. Most states recognize out-of-state placards, although California requires you to obtain a temporary placard if you're just passing through.
TL;DR (Too Long; Didn't Read)
Most states recognize the handicap placards from other states. Some states, such as California, may require you to get a temporary placard while you stay there.
Obtaining Handicap Placards
Placards are issued by the state's motor vehicle department, on a doctor's recommendation. The placard does not need to be for the driver; someone with a qualifying condition can use it as a passenger. The placard has expiration information printed on it. A driver who has a handicap license plate may also receive one placard. If you're visiting California, you'll need to get a temporary placard for travelers, which you'll need to keep visible in the car when you park in a handicap spot.
Qualifying Medical Conditions
Some of the qualifying conditions include heart problems, use of oxygen, inability to walk at least 200 feet without resting, or the use of a cane, crutches or prosthetic. A person who uses a wheelchair also qualifies for a placard.
International Symbol of Accessibility
The International Symbol of Accessibility is easily recognized. It's a stick figure of a person in a wheelchair, in white on a blue background for signage. The handicap placard has the symbol on it. The designated spaces also have a sign with the symbol. The sign must be at least 60 inches off the ground, and it must be clearly visible. Most handicap parking spaces have the International Symbol of Accessibility painted on them in either blue or white, although it's not required by law.
Handicap Parking Spaces
There must be one handicap space per 25 spaces in a parking lot. The space needs to be by a crosswalk, and be near the building's most accessible route. The spaces need to be at least 60 inches wide, and have room for an aisle to get in and out of the vehicle. One handicap space out of eight needs to be van accessible, at least 98 inches wide. Van spaces need to be marked on the sign. Cars with a handicap placard or license plate are allowed to park in van spots.
Abuse of Handicap Placards
It is illegal to park in a handicap space without a handicap placard or license plate. It's also illegal for someone with the proper placard or plate to park in a designated space without the person to whom the privilege was issued. Fines for parking illegally in a handicap spot vary from state to state. The average is $250 for a first violation. Some counties impose even harsher punishment. Abuse of handicap parking laws in some counties can run fines of $500 or more, and even include community service.