*2.3. Manual Radar Survey Procedure*

Manual survey locations were established on each approach to the treated junctions and mid-link on the curve and merge sections. A pre-survey reconnaissance visit was undertaken to determine the exact location of each site in order to consider the health and safety of the enumerator, the covertness of the survey, and the potential accuracy of the recording. Specific concerns at each location included the angle of the radar beam, the depth of field, the field of view to the traffic flow, and the presence of objects which may have resulted in unwanted radar reflection. In total, 21 sites were established. Prior to each survey being undertaken, weather forecasts and traffic-incident information were checked and rescheduled if conditions were considered sub-optimal. This ensured that weather and unexpected traffic conditions were controlled for all surveys being undertaken during 'neutral' conditions, i.e., good visibility, no precipitation, light or no winds, generally dry road surfaces, and normal traffic flows. A typical radar survey arrangement is shown in Figure 2.

**Figure 2.** Typical radar speed survey arrangement.

To provide an adequate sample size, the surveys were carried out for a minimum of one hour at each location, or longer if a minimum threshold of sixty vehicles had not been reached. Only vehicles subject to free flow conditions were recorded, defined as where a headway of greater than three seconds existed, as used in other similar studies (e.g., [7]. Wherever practicable, every vehicle travelling in such conditions was recorded. The speeds were recorded directly into a spreadsheet during the survey itself, with cosine corrections applied if local factors required a larger angle of measurement. Each site was surveyed once during the hours of daylight and once during the hours of darkness. The exercise was repeated three times over a 30-month period: before installation of the active road studs, six months after installation, and two years after installation. The surveys were undertaken in November or February to ensure that sunset times and ambient lighting conditions were controlled to be as similar as possible.

#### *2.4. Additional Data Sources*

To investigate other road factors that may influence speed, a combination of Ordnance Survey (UK national mapping agency) vector data, and visual observation of the infrastructure was used. The mapping data was used to derive the vertical and horizontal alignment of the route along with the carriageway widths at each speed-survey location. The gradients on the approach to the survey sites were derived from the vertical alignment profile and presented as a percentage; uphill gradients were recorded as positive and downhill gradients as negative, respectively. Curvature values were taken initially as an absolute centreline radius in metres; these were subsequently recoded into three binary variables corresponding with the desirable minimum, one-step, and two-step relaxation design values from the UK highway design manual [27]. Any radii exceeding 2880 m were considered tangent sections. The carriageway widths were measured at either end of the zone covered by the radar gun; these values were then averaged and divided by two to provide an effective carriageway half-width for the survey location.

Visual inspection at each survey site provided the posted speed limit (PSL), the presence of street lighting, the presence of 2 + 1 carriageway configuration, the locations of junctions or merges, and other features which may affect speed choice. Binary variables were created to represent each feature accordingly. Two further variables were created with respect to the presence of enforcement cameras; one represented the distance since the last camera with the direction of flow, the other the distance since the last camera passed facing either direction. A variable was also coded representing the length of the active road stud installation on the approach to the survey site. Each speed case recorded during the surveys was appended with the previously described variables corresponding to the survey site in question; binary variables were also included for whether the case had been recorded during daylight or darkness and whether it was before or after the implementation of the active road studs.
