I'm still looking for decent primary sources, but this Guardian article contains some fairly revealing tidbits:
In 2002, Ireland introduced a 15 euro cents tax on each plastic bag – the so-called "plastax" – and within a few months a 90% reduction in the number of bags being used had been recorded. But the scheme has had its critics.
While it was true that the tax led to a dramatic drop in the number of bags being handed out in shops, it also triggered a 400% increase in the number of bin liners and black refuse bags being purchased.
The tax also encouraged an increased reliance on paper bags which, according to a number of life-cycle analysis studies that have compared the environmental performance of various types of bags, require more energy to manufacture and release more greenhouse gases when degrading following their disposal.
And while it is commonly accepted that plastic bags are a genuine blot on the landscape (and seascape), they only represent a tiny fraction of the waste stream by weight or by volume. For example, in the US they account for less than half a percent of domestic refuse.
Goodall says that the various efforts to reduce the use of plastic bags – be it through government legislation or the voluntary efforts (spurred on by high-profile campaigns by the likes of the Daily Mail) by supermarkets to reduce their customers' reliance on such bags – are invariably littered with unintended consequences. As has been seen in Ireland, plastic bag taxes often lead to a rise in the number of bin liners being purchased. "This plastic is much thicker and will prove to be a greater environmental hazard than thin plastic bags," he says.
There are also growing rumbles of concern in San Francisco, which, in 2007, became the first city in the US to introduce a plastic bag ban. An investigation by the San Francisco Weekly earlier this year found that in the period since the ban was introduced there had actually been a slight rise in the number of plastic bags picked up off the city's streets.