The Public Health Interventions Advisory Committee (PHIAC) took account of a number of factors and issues when developing the recommendations.
3.1 PHIAC agreed that there is a moral imperative to protect children and young people, including on the roads. This includes addressing the behaviour of drivers through a variety of approaches.
3.2 Although engineering measures are important in preventing casualties, PHIAC discussed the importance of other factors. These included education, enforcement, and changing the percentage of journeys undertaken by car, public transport, on foot or by bicycle (modal shift). Engineering, education and enforcement activities are likely to be synergistic.
3.3 Methodological difficulties may make it hard to be clear about what an intervention has (or has not) achieved:
Engineering measures are not commonly assessed using trials.
The overall downward trend in injuries makes comparisons over time difficult.
The numbers of people killed or seriously injured are relatively small, so it is difficult for studies to be adequately powered to determine whether an intervention has been effective.
There is a lot of work to prevent injuries, both locally and nationally, which may add to the difficulty of identifying effective elements of interventions.
The diffuse nature of some interventions, often involving multiple components, makes comparisons between them difficult.
Interventions may be designed to achieve a range of outcomes.
Interventions are generally designed to reduce casualty rates for all road users rather than just children and young people.
3.4 Much of the evidence considered was from the UK and so was deemed applicable for England. However, PHIAC was aware that older UK publications might be less applicable, because changing political, cultural and economic backgrounds can alter the effectiveness of interventions. Nonetheless, it noted that the evidence consistently suggests that engineering measures to reduce traffic speed generally do reduce collisions and deaths or injuries among children and young people.
3.5 For inclusion in the reviews, evidence needed to provide data on injuries to children and young people. If data on speed was also provided, this was included. However, the literature relating to speed alone has not been considered in this work. Similarly, studies that did not provide an analysis of injuries to children and young people aged under 15 were not included.
3.6 PHIAC noted that pedestrians are much more likely to be killed in collisions at higher speeds.
3.7 For several types of intervention identified in the scope for this work, the reviews either found no evidence (for instance for woonerven and 'naked streets') or found no impact on injuries (for instance for 'home zones' – where injury reduction is not the primary purpose). Therefore these interventions do not appear in the recommendations.
3.8 Engineering measures may have other outcomes (both positive and negative) apart from helping to prevent injuries. These include noise, damage to buildings or vehicles (from vibration and the impact of vertical traffic-calming features) and air pollution (including CO2 emissions). Changes in behaviours influenced by engineering measures may also be related to health outcomes, for instance increasing levels of physical activity by supporting cycling and walking or encouraging greater social contact.
3.9 Changes to the physical environment can have unintended consequences that may disadvantage some groups. For example, changes that remove physical features (such as the distinction between pavement and road) might increase uncertainty on the part of motorists, and so promote a safer driving style. However, they might also make negotiating a street more difficult for people with a visual impairment.
3.10 PHIAC noted that the attitudes of communities and drivers to speed reduction measures are important. Drivers may be more accepting if they can see the point of speed restrictions (such as those near schools – although these areas may not, in fact, have significant injury rates).
3.11 Economic analysis in NICE guidance generally consists of an estimation of the cost per quality-adjusted life-year (QALY) gained. This enables a comparison with what is deemed to be value for money in the health service. However, when assessing road transport interventions other approaches may be more appropriate. In particular, the Department for Transport uses cost–benefit analysis taking a 'broad societal perspective' to assess value for money. Net present value (NPV) is used to determine the total monetary benefit of an intervention less its costs (compared with an alternative intervention) when discounted to its present value. A positive NPV occurs when the sum of the discounted benefits exceeds the sum of the discounted costs. As the costs fall on the transport sector, it is more appropriate to compare cost effectiveness with other transport interventions using a method followed by that sector. This is in line with 'Social value judgements: principles for the development of NICE guidance'.
3.12 Enforcement strategies were not covered in the scope of this guidance. PHIAC noted that NICE's guideline on unintentional injuries: prevention strategies for under 15 was considering enforcement.
 A woonerf (plural woonerven) is a Dutch term for a street where pedestrians and cyclists have priority over motorists. Motorised traffic is restricted to walking pace.
 Roads cleared of markings, signage and pedestrian barriers.