*3.2. Change in Route Mean and Mean Excess Speed*

The significance testing for the changes in the global mean speed along the route is shown in Table 2 for the 60 mph and 70 mph posted speed limits, respectively. The F and t values for Levene's test for equality of variance and Welch's test for equality of means are shown, respectively, along with the associated probability (*p*) values and the degrees of freedom (df). For the 60 mph posted speed limit sections, the changes in the calculated overall mean speed (Δ<sup>−</sup> v) after the implementation of the active road studs were negligible. During the hours of darkness in both scenarios, the mean speeds were reduced compared with the situation before installation. In the hours of daylight, a negligible reduction was recorded six months after installation, followed by a small increase after two years. No scenario was recorded with a significance level less than 0.05; therefore, in terms of the global mean speeds of the route, the null hypothesis cannot be rejected for the 60 mph posted speed limit sites.

For the speed distributions at the 70 mph posted speed limit sites, a reduction in mean speed of 1 mph overall was apparent six months after the treatment, or 1.5 mph during the hours of darkness. This reduced in magnitude slightly after two years, but a decrease was still apparent. In the former case, the results were recorded at a significance level of less than 0.05, and the null hypothesis may be rejected. Furthermore, a statistically significant change in variance could be observed during the six months after the treatment in all lighting conditions, although this was no longer the case two years after implementation.

The significance testing for changes in mean excess speeds (i.e., speeds greater than the posted limit) is shown in Table 3 for the 60 mph and 70 mph posted speed limit sections, respectively. The changes in the mean excess speed after the implementation of the active road studs were generally negligible. The differences in any change in mean excess speed between light and dark conditions were found to be broadly similar in both magnitude and direction in each scenario. Only the change in mean excess speed in the 60 mph scenario after two years approached significance; for the other scenarios, the null hypothesis cannot be rejected.


**Table 2.** Change in mean speed by posted speed limit.

<sup>a</sup> Denotes significant result (*p* < 0.05).


<sup>a</sup> Denotes significant result (*p* < 0.05).

#### *3.3. Change in Mean Speed by Site*

Changes in the local mean speed on an individual site basis along the route were assessed; the results of the analysis are shown in Table 4. The results suggest that the null hypothesis cannot be rejected at most sites. Five sites showed statistically significant changes in mean speeds immediately after treatment, increasing to nine sites after two years. The changes were mixed in direction and magnitude. Immediately after the stud installation, a decrease in mean speeds was seen at four sites and an increase at one. Two years after the installations, six sites had decreased in mean speed and three had increased. The largest statistically significant decrease in mean speed was 3.5 mph and the largest increase was 4.6 mph.


**Table 4.** Change in local mean speed by site.

<sup>a</sup> Denotes significant result (*p* < 0.05).

#### *3.4. Infrastructure Survey*

The results of the infrastructure survey of the 21 speed-survey sites are shown in Table 5. The table shows a reasonable mix of physical characteristics across the survey sites. During the initial analysis runs, the results from sites 3 and 4 were found to be inconsistent with the other sites. From a legal perspective, these two sites are technically 70 mph dual carriageways. However, whilst the road is physically divided by a central reserve, only single lanes exist in either direction due to pavement hatching markings. The site is located at the transition between the true single carriageway and the dual carriageway sections. Upon inspection, the speeds recorded at this site appeared to be closer to the speeds commensurate with a single carriageway. On this basis, for the purposes of subsequent analysis, the site was recategorized as a 60 mph single carriageway, which was considered to be more representative.

Descriptive statistics for the physical infrastructure recoded into binary variables, along with the retained continuous variables are shown in Table 6. The results presented are disaggregated into the two posted speed limits with the three survey periods (before, six months after, and two years after) combined. The mean and standard deviation of the speed varies across the two scenarios, but the same statistics for the physical characteristics remain broadly the same. The similarity of the sample size at each individual site is incorporated within the survey design and suggests no single site was overrepresented. Entries have been omitted where the variable is constant under certain circumstances. Each of the 60 mph sites featured a junction, and there were no 70 mph survey sites featuring a desirable minimum radius curve, street lighting, or a merge. The2+1 carriageway is only applicable to 60 mph roads so does not feature as a variable in the 70 mph model. The darkness indicator is only applicable to the 'all conditions' dataset, as it takes a constant 0 or 1 value in the light-only or dark-only models.


**Table 5.** Speed survey site characteristics.

<sup>a</sup> Site featured central reserve but single running lanes—assigned as single carriageway with 60 mph PSL. <sup>b</sup> Radii greater than 2880 m were assigned as tangents.

**Table 6.** Variable descriptive statistics.


#### *3.5. Correlation Analysis*

The Pearson's product-moment correlation coefficients between variables are shown in Tables 7 and 8 for the 60 mph PSL and 70 mph PSL datasets, respectively. In the 60 mph dataset, small positive correlations between measured speed, the presence of a desirable minimum curve (r = 0.11, *n* = 6299, *p* < 0.01), and the distance since the last enforcement camera in any direction (r = 0.18, *n* = 6299, *p* < 0.01) were found. In the 70 mph dataset, small positive correlations between measured speed, the presence of a two-step relaxation curve (r = 0.16, *n* = 2886, *p* < 0.01), and the length of treated approach (r = 0.18, *n* = 2886, *p* < 0.01) were found. A small negative correlation was found between the measured speed and the presence of a two-step relaxation curve (r = −0.11, *n* = 6299, *p* < 0.01) in the 60 mph dataset. In the 70 mph dataset, a small negative correlation was found between measured speed and the presence of a one-step relaxation curve (r = −0.13, *n* = 2886, *p* < 0.01)


**Table 7.** Correlation of measured speed with road factors (60 mph PSL).

*n* = 6299; — variable constant for all surveyed sites; <sup>a</sup> 0.99 level of confidence; <sup>b</sup> 0.95 level of confidence.

**Table 8.** Correlation of measured speed with road factors (70 mph PSL).


*n* = 2886; — variable constant for all surveyed sites; <sup>a</sup> 0.99 level of confidence; <sup>b</sup> 0.95 level of confidence.

In terms of the other variable pairs, in the 60 mph PSL dataset, only one pair was found with r > 0.7. In the 70 mph PSL data set, four variable pairs were found with r > 0.7. On this basis, there was little evidence of multi-collinearity; hence, the full variable datasets were taken forward for further linear regression analysis.

#### *3.6. Linear Regression Analysis*

The linear regression models are shown in Table 9 for the 60 mph PSL and the 70 mph PSL datasets. In the 60 mph PSL model, the largest positive increase in speed was found to be associated with the presence of merges, with a 9.3 mph increase suggested. Other variables contributing to increased speeds included the presence of darkness, the presence of 2 + 1 carriageway, and the increasing distance from the last enforcement camera. Decreased speeds in the model were associated with the presence of street lighting, uphill gradients, and the presence of curves with radii smaller than the desirable minima. No dual carriageway sites featuring street lighting, a merge, or a curve of desirable minimum radius were part of the installations monitored. The statistically significant explanatory variables are therefore fewer in the 70 mph PSL model. The presence of darkness and the increasing half-width were associated with increases in speed. In terms of the active road studs, their specific presence was found to reduce speeds, whereas increases in the length of installation were found to increase speeds.


**Table 9.** Linear Regression Model.

— statistically insignificant variable (not included for estimation in model specification); <sup>a</sup> 0.99 level of confidence; <sup>b</sup> 0.95 level of confidence.

#### **4. Discussion**

Although the causal relationship between speed and the probability of a crash remains subject to debate, greater severities of injury can be expected at higher speeds [4]. As a result, practical interventions which may result in increases in speed should be viewed with a degree of concern. Even measures which are intended to improve the safety of the driver– vehicle–road system may result in undesirable risk compensation by drivers and have a net opposite effect. For example, this phenomenon has been associated with interventions such as seat belts, anti-lock brakes, and the implementation of street lighting [29–31]. In the specific case of solar-powered road studs, if increases in driver confidence [17] were to translate to a corresponding increase in mean speed, this could result in increased crash rates [9–14]. The relationship between active road studs, other road features, and speed is therefore of considerable interest.

This research found that in the 60 mph PSL sections when the route was taken as a whole, only very negligible reductions in mean speed were recorded, none of which approached significance. Essentially, from this it would appear that mean speed appears to be unaffected by a change from passive to active road studs on such roads, a result which reflects that found in the previously described simulator study [18]. No simulator study has looked at a 70 mph dual carriageway route, and hence, the results found in this respect are believed to be new. In this case, a statistically significant reduction in mean speed of 1.5 mph was found during the hours of darkness six months after the treatment. Some reduction in this figure occurred after two years, and it was no longer significant at the chosen alpha of 0.05 (although notably it still was at α = 0.10). Decreases in variance suggest that speeds become slightly more homogeneous, and large fluctuations are less evident, which is a positive outcome when the safety effects of speed differentials are considered [5,6].

In terms of speeding drivers, the improved visibility from active road studs does not appear to change the mean excess speed in either the 60 mph or 70 mph PSL sections. Other research has suggested that reflective delineation helps drivers' speed awareness at night [32]. Given the additional information provided by the active studs, it seems probable that speeding drivers are doing so consciously with full knowledge of their actual speed. Whilst addressing speeding drivers may be an emotive issue with members of the public, failure to do so does not necessarily mean that safety benefits will not be achieved. Although, as discussed previously, the link between speed and individual crashes is a matter of debate, the link between mean speed and crash rate seems much clearer [8]. For

example, application of the power model [11] would suggest the reduction in mean speed during darkness measured here could reduce the number of fatal crashes by up to 10%.

Whilst significant global mean speed changes were restricted only to the 70 mph PSL sections of the route, mean speed changes at selected individual sites across both speed limits were found. No site could realistically be considered the same; each had its own geometric and topographic characteristics. In many cases, the implications of such characteristics are intuitive. For example, it would be reasonable to expect drivers to traverse curves of one- and two-step design relaxations at a slower speed in general than those of a desirable minimum radius, if only in the interests of occupant comfort. On this basis, it would appear that any relationship between mean speed and road features is a complex one, where the active road stud may be a contributory variable in certain circumstances. The subsequent regression analysis undertaken provides some insight into how the active road stud, along with other variables, may be associated with mean speed.

In the 60 mph PSL regression model, the greatest effect on speed was the presence of a merge. This could be a result of drivers travelling faster than usual to complete passing manoeuvres before the end of overtaking lanes. Whilst this result was statistically significant, it is noted that only two merge sites existed in the dataset, one of which was on a single carriageway divided by a central reserve, which is a relatively unusual arrangement. On this basis, it is suggested that other factors may contribute to this increase. Variables contributing to reductions in speed, particularly those related to gradients and curves, would seem intuitive; it seems quite possible that the speed of the vehicles may be affected by increased engine strain on the gradients and the discomfort and lower visibility experienced on the sharper curves. Where street lighting is not present, the increase in speed in the hours of darkness found in this study is consistent with other research findings [29,33].

In terms of the active road studs, the presence of the studs was found to be insignificant in the 60 mph PSL model. However, the length of a treated approach variable was retained. The decrease in mean speed associated with an increased length of stud treatment is a potentially interesting finding. However, it is noteworthy that the studs were installed only on the approaches to junctions, with the length of installation being proportionate to the importance of the junction itself. On this basis, it could be that drivers may, at least partially, have already adjusted their speed in anticipation of the conditions ahead, and the association with stud treatment length was merely coincident. Given that all the 60 mph PSL sites were at junctions, unfortunately it was not possible to control for this possible effect with the data gathered.

In the 70 mph PSL model, again the presence of darkness was associated with higher speeds, more so in fact than the former. Significantly, the presence of the active road stud appeared to influence speed, with a reduction of 1 to 1.5 mph being implied. In the case of the length of the stud installation, the opposite to the 60 mph PSL model occurs; in this scenario, speeds increase as the length of treated approach increases. It is known that the dual carriageway sections of this road are mostly used to provide overtaking opportunities. On this basis, it may be that longer sections of dual carriageway are more attractive for overtaking and therefore result in higher speeds, which could explain this apparent pattern.

The other variable of significance in the 70 mph PSL model was road half-width. In this case, increases in half-width appeared to result in increased speeds. It has been suggested that the additional steering workload, along with a better perception of speed, is associated with slower speeds in narrower lane widths [34,35]. Such association could be expected during hours of daylight but less so during hours of darkness, where a driver's field of view and depth of field are restricted by the capability of the vehicle headlights. It could be that the presence of active road studs might enhance a driver's awareness and perception of speed, leading to this association.

Some limitations in the present work are acknowledged. Most of the survey sites were located on the approach to junctions as they were the focus of the original installation. It could be that drivers already moderate their speed when in such circumstances, in which case any change in mean speed may be suppressed. The work here was also entirely random in its sampling, and specific vehicle types were not recorded. Whilst this means the results are likely to be valid for typical traffic composition, the effects may be different between vehicle categories. This study also controlled for weather, with all the results obtained during benign conditions. One aspect in which active road studs are thought to be of particular benefit is during times of poor visibility. On this basis, their effect on mean speeds under such conditions could be worthy of further investigation.

#### **5. Conclusions**

The aim of this research was to measure the choice of speed by drivers, using realworld rural junctions and links and to determine whether changes in the speed of vehicles may be associated with the installation of active road studs. The null hypothesis stated that drivers do not change their speed in the presence of active road studs, and there is no difference in mean speeds before and after installation. Based on these findings, looking at the entire route, a universal rejection of the null hypothesis across all tests is not possible; there is no consistent correlation between mean speed and the presence of the active road studs at all sites. However, there appears to be some specific situations in which the null hypothesis could be rejected. Most notably, on the 70 mph PSL dual carriageway sections, the presence of studs may contribute to mean speed reduction as part of a wider group of variables.

The results of this work are likely to be of interest to road safety engineers, specifically as the improved visibility resulting from the installation of active road studs appears unlikely to result in any corresponding increase in the mean speed of vehicles. This is a positive finding, as it means that any other safety benefits found, such as improved driver confidence or better lane discipline, would not be offset by the risk of greater crash severity through increased speed. On higher-speed dual carriageway sections, the presence of the stud may even result in a reduction in mean vehicle speeds. In road safety terms, this could mean that the severity of crashes may be decreased in the presence of the stud. This would appear to be a particularly positive outcome, given that these are the highest speed sections—and therefore potentially the highest crash severity—on the road network.

**Author Contributions:** Conceptualization, R.L. and J.C.; methodology, R.L.; software, R.L.; validation, J.C. and G.F.; formal analysis, R.L.; investigation, R.L.; resources, R.L.; data curation, R.L.; writing—original draft preparation, R.L.; writing—review and editing, R.L., J.C. and G.F.; visualization, R.L.; supervision, J.C. and G.F.; project administration, R.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Amey for facilitating the inspection of the installation of the works during construction and Transport Scotland for the provision of the background information regarding the scheme.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

