Visibility splays define the area that must be kept clear so that drivers at a junction can see approaching traffic. The key factor influencing splay requirements is the speed of vehicles on the main road.
Visibility is most commonly assessed for drivers exiting a junction, but checks may also be required for forward visibility, for example around bends.
Guidance on how visibility splays should be measured and applied varies, with differences between national standards and local authority approaches. Assessing visibility therefore requires careful consideration of the relevant guidance, appropriate measurement of vehicle speeds, and a site-specific appraisal to ensure that requirements are applied correctly.
When do you need to test
Visibility splays are a key consideration in almost all highway junction designs. In straightforward cases, splays can sometimes be plotted without specialist input. However, many sites require more detailed assessment, including accurate plotting, speed surveys and interpretation of guidance.
Visibility is often assessed at an early stage when appraising access options for a development (see access appraisal & feasibility studies), but issues are also frequently raised during the planning process. In these cases, resolving visibility constraints can be critical to demonstrating that a safe and suitable access can be achieved.
LTP has extensive experience working with highway authorities across the UK, including National Highways and Transport for London. We understand how different authorities interpret and apply guidance, and tailor our advice accordingly.
LTP has undertaken hundreds of access appraisals across a wide range of development types and scales, allowing us to support schemes of all kinds.
Vehicle speed data is often required to inform visibility splay assessments, and obtaining this data can add cost and time to a project. Where possible, we identify suitable existing data, and when new surveys are required, we work with trusted suppliers to ensure an efficient and proportionate approach. Depending on the circumstances, LTP’s in-house staff are also able to undertake manual radar speed surveys, which can be the preferred survey type.
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