I used to favour the use of biodiesel, and spent some time researching it. Creating your own is quite easy, but you need to be meticulous while you go about it. The issues are things like removing bids of food if it is second hand oil, and removing water, and safe use of caustic chemicals.

My reasons for changing from using biodiesel are as follows:

Sure it's a renewable source of energy, but at what cost to the planet and its people - rip out tropical rainforests to plant palm oil plantations does not seem the right way to go otherwise it's a case of using cropland and using it for the self-indulgent purposes of rich westerners versus food for the starving millions.

Bio-diesel would possibly be ok in super frugal internal combustion engines, but to using it to run humvees, or SVUs or as here in Australia 4WD (4 wheel drives) is in my opinion, self-indulgent greediness.

I think biodiesel should only be sourced from waste products such as fat for deep frying food, should be reserved for use of public transport and emergency services. Everybody else should walk or ride a bike.

Another reason for not using biodiesel, is ultra-fine particulate matter. This is caused by all internal combustion engines (ice), and the particles are so fine they go directly through the lung walls into the blood stream, can interrupt the heart beat and affect neural transmission.

In terms of things like rising obesity levels, I think a major disruption to the ownership and use of personal use vehicles would be for the good of all. Various bits of research show that we need to walk a minimum of 5 kilometres (3 miles) a day to maintain a healthy weight and even more if we want to be slim. Rather than going to the gym or setting aside time for jogging a public health policy is better served by changing transport infrastructure that incorporates this level of activity into normal daily life. A combination of walking to the bus or train stop and using public transport to and from work would be one way to do this.

It would be tempting to say use electric cars. At this stage of global warming issues this is folly. Even retro fitting existing vehicles would emit tons of carbon dioxide from the manufacture of the electric engines and storage batteries alone, and if electricity from coal gas or nuclear is used to recharge those batteries the whole exercise is pointless.

Best answer - get a bike and lobby for good public transport and better transport policies.