Can plastic be used as fuel for vehicles?
There are many ways to recycle different types of plastic, and conversion to liquid fuels suitable for internal combustion engines is currently done on an industrial scale. The key to converting the carbon contained in long-chain polymers that make up the plastics is called pyrolysis, which is essentially a way to thermally decompose these products into shorter chain hydrocarbons suitable for use as liquid (or even gaseous or solid) fuel.  According to this Wikipedia article:

Anhydrous pyrolysis can also be used to produce liquid fuel similar to diesel from plastic waste, with a higher cetane value and lower sulfur content than traditional diesel.

An example of a current commercial waste-plastic-to-fuel operation is the Canadian corportion Plastic2Oil, Inc., who claims to &quot;convert waste plastic to ultra clean oil&quot; in their 250,000 gallon fuel production and blending facility.
The following is an excerpt from the description of the patent-pending process used by Plastic2Oil:

The processor accepts unwashed, unsorted waste plastics, composites and commingled materials. Although many sources of feedstock are available, we are focusing initially on post-commercial and industrial sources, since these are readily available in large supply, and present a cost-effective solution for companies. We currently have several companies providing plastic feedstock.
The hopper is loaded with up to 4,000 lbs. of plastic feedstock per machine per hour using a forklift. The plastic is loaded into the pre-melt reactor by a continuous conveyor between the hopper and the reactor. The plastic is then heated using the off-gases produced by the process. After the plastic has been liquified in the pre-melt reactor it passes through a solids-liquids separator before going into the main reactor.
In the main reactor, the liquified plastic hydrocarbons are cracked into various shorter hydrocarbon chains and exit in a gaseous state. Plastic2Oil's proprietary catalyst and unique process engineering enables us to capture an average of 86% of the hydrocarbon content of plastic. The Petcoke residue produced at this stage (approx. 2-4%) remains in the processor chamber and is automatically removed while the processor is in operation.
From the main reactor, the gases that are fuel oil and diesel are condensed and separated, then proceed into temporary fuel tanks. All of the gaseous &quot;light fractions&quot; or off-gases (approx. 10-12% of process output), such as methane, ethane, butane and propane, exit the system and are used to fuel the furnaces.

Note: I have no affiliation whatsoever with Plastic2Oil or their industry. I chose their company as an example largely at random, or more accurately, due to their very usable and easily found website.