Just over sixty years ago a LSHTM scientist, Jerry Morris, is said to have „invented exercise‟ (Kuper 2009). The evidence he used “to show that exercise can extend your life” (Kuper 2009) came not from gymnasia or playing fields, as one might expect, but from observations made while riding London‟s buses. Out of this surprising setting Morris showed that „unavoidably active‟ bus conductors had substantially fewer heart-attacks than their „protypically sedentary‟ bus driver counterparts (Kuper 2009). This despite these research subjects sharing similar social class backgrounds. On the fieldwork carried out by him and his team, Morris recalls that they “spent many hours sitting on the buses watching the number of stairs they [the conductors] climbed” (Kuper 2009), and so it seems fitting that six decades on a team of LSHTM researchers will, in the name of public health, be acquainting themselves with the activities taking place on London‟s buses once more. This time around the primary interest for the researchers is in neither the health outcomes of bus drivers nor of long-departed bus conductors. Rather, as the scope for transport policy to mediate health-promoting activities is realised, and while at the same time substantial public spending cuts loom large, this study will focus on the relationship between fare exemptions for younger and older patrons of London‟s public bus network and public health in the capital. As with Morris‟s 1949 research, the current „On the buses‟ researchers will draw, in part, on data derived from „natural‟ settings to make their claims. In the present study, the researchers are interested in the health-promoting or health-damaging consequences that can be attributed to policy interventions that were directed towards reducing the financial costs of travel for young people in London. The specific interventions concerned are the removal of bus fares for 12-16 year-olds in September 2005 and the removal of bus fares for 17 year-olds in full-time education September 2006. These fare exemptions for young people were introduced during the tenure of the previous Mayor of London, Ken Livingstone (Mayor 2000-2008), who was known for introducing public transport subsidy schemes during his time as leader of the Greater London Council (GLC) in the 1980s.2 When the first (2005) intervention concerned was launched, secondary school-aged children had paid a reduced, 40p flat fare for journeys on the London bus network.3 As well as grant the cardholder unlimited free travel on all buses and trams displaying the London Buses symbol (both within and just outside London (see TfL 2010)), Zip Cards also act as conventional „Oyster‟ cards and can be loaded up with pre-pay or travelcards for the cardholders to use other parts of the TfL network (Tube, DLR, London Overground and most National Rail services operating in the capital) at a variety of discounted rates (see TfL 2010: 6-11). When the fare exemptions with which we are concerned were unveiled, the stated rationale for universally eliminating bus fares for young people in London was to help them to continue studying, improve employment prospects and promote the use of public transport” (TfL 2006: 7). That is, it was aimed first and foremost at mitigating the potential social exclusion effects for young people of fare-based urban transport systems (see Social Exclusion Unit 2003). As it has been stated more recently on the TfL website: Granting young people free travel is part of the Mayor's strategy to embed more environmentally sound travel habits from an early age while helping young people to unlock education, sport, leisure and employment opportunities (TfL 2007). By removing any need to pay, at the point of use, for travel on buses, the argument went that young people would be better (and more equally) able to access goods and services (schools, libraries, leisure facilities etc.) and so reduce the chances of their suffering from transport poverty. At the same time, it was hoped that by encouraging bus use from an early age more environmentally sustainable travel practices would become ingrained. The effectiveness of the Zip Card scheme in relation to these posited outcomes is still up for debate, though there is some broader evidence for the value of such interventions (e.g.Ogilvie, Mitchell et al. 2006). For us, however, our interest lies not in the success or not of the Zip Card in relation to its initial objectives, but rather in the public health impacts of these fare exemptions. That is, this study addresses the consequences for the broader public health of a social policy that was not introduced with health in mind in any explicit way but which may significantly shape the health outcomes of Londoners all the same.