Road Link Speed Profiling

Road roughness is a term used to quantify the relative user comfort on a road link (COTO, 2007). Roughness is seen as a holistic measure of several road conditions such as rutting, cracking, potholes and undulations (COTO, 2007). The international roughness index (IRI) is used to measure roughness. IRI is a component of the longitudinal profile on a road, which is experienced in the wheel path of the travelling vehicle (Archondo-Callao, 1999).

A direct relationship exists between speed and roughness (Sayers, et al., 1986). This would imply that speed is also a measure of several road conditions just as roughness is. In the guideline from Sayers et al. (1986), four different classes of roughness measurement methods are described. Each measurement method varies in accuracy, expense and the required measurement instruments. Class 4 was selected as the method of choice for the proposed accessibility measure. The method is labelled as “subjective ratings and uncalibrated measures”. A class 4 roughness measurement is achieved by either a ride experience or a visual inspection of the road (Sayers, et al., 1986). Measurements from an uncalibrated instrument can also be used in this class. This class differs from the other classes as it does not require specialised personnel or measuring equipment but this also implies that there is a much greater margin of error. Table 3.20 shows the correlation between average speed and average IRI by road class as given Van Zyl (2016).

Table 3.20. Passability classification, average road speeds and IRI for gravel roads (van Zyl, 2016).

Road class Passability (impassable days/ year) Mobility Average Speed (km/hr) IRI Average (mm/m) R1 2 80-100 < 5 R2 2 80-100 7.5 – 5 R3 3.5 60-80 10 – 7.5 R4 3.5 45-60 13 – 10 R5 3.5 < 35 15 – 13

Using Table 3.20, it was concluded that for all road links, the average classification speed be given as that of road class 4 which is a mobility speed of 45 km/h to 60 km/h.

3.5.2.1 Measuring the speed profiles

The method of ride experience was used to measure the change in speed along the longitudinal path of the roadway. The method was supplemented with a Garmin Nuvi 2597LMT handheld GPS receiver. The receiver was used to obtain a profile of speed along the distance travelled.

Garmin GPS receivers are accurate to within 15 m 95% of the time with a clear view of the sky (Garmin International, 2016). Generally, users receive signal at an accuracy of 5 – 10 metres under normal conditions (Garmin International, 2016). At the investigation locations from Mangwele to Tshianane Secondary School, the GPS could have had accuracy closer to 5 metres given the clear sky, which was present, and the absence of any obstruction in the sky as well.

The car used was a double cab 2014 4X2 Ford Ranger with an automatic gearbox. The vehicle started at Mangwele. driving in the direction of the schools. The car was driven to a maximum comfortable speed at all times. This measurement was carried out in three phases. The first phase consisted of driving from Mangwele to

Intersection 1. The second phase, from intersection 1 to Ramabulana Secondary School and Straight Hardt Clinic. The third phase consisted of driving from intersection 1 to Tshianane Secondary school. The vehicle was brought to a complete stop at the beginning and at the end of each driving phase.

The vehicle used had a coil-over-strut wishbone front suspension and a leaf rear suspension, which has been reported to give a compliant ride (Motoring, 2014). The type of rear suspension in the vehicle is known to allow for a more comfortable ride when laden (not to capacity). The test vehicle used in the research was unladen. The test vehicle also had wider aftermarket tyres that also could have affected the ride quality. The exercise was conducted with all this in knowledge.

The handheld receiver could not be set up to record the speeds at set intervals and recorded the speed at what seems to be random time intervals called legs. Table 3.21 shows leg summaries of the three phases recorded. Phase 1 consisted of measurements take from Mangwele to intersection 1 as shown in Figure 3.5. Phase 2 was measurements from Intersection 1 to the end of road link 3. Phase 3 was measurements from Intersection 1 to the end of road link 4.

Table 3.21. Recorded leg characteristics from GPS Receiver for subject road links in phases. Phase Number of legs Average Leg length (m) Average Leg time interval (seconds) Maximum Leg time interval (seconds) Minimum Leg time interval (seconds) Phase 1 257 44.11 6 16 1 Phase 2 24 46.63 7 13 1 Phase 3 43 116.37 9 16 1

Depending on the road class, the minimum mobility average speed should be achieved comfortably.

3.5.2.2 Segmentation according to speed profiles

To identify road sections that require maintenance and rehabilitation, road administrations use homogenous sections (Thomas, 2004). It is important to analyse the data from the speed profiles in order to identify these homogenous sections, for which a mean speed value can be defined which presents significant differences compared to the averages of other adjacent sections (Cafiso & Di Graziano, 2012).

The method of cumulative differences was used to identify these homogenous sections. The method used is recommended in the AASHTO guide for design of pavement structures (1993). It compares the sequence of the actual sums of the measured speed data with the sums that would result from simply adding the averages (Thomas, 2004). The cumulative difference value is given by Equation 3.15 (Thomas, 2004).

!_" = " $_% − '$

%() (Equation 3.15)

where

k = represents the data sequence (k =1,…,n)

+ =,

- +.

- .(,

When the series, z1, z2,….,zn, is plotted as a function of the distance along the road,

unit boundaries occur where the gradient of the function changes signs between being positive and negative (AASHTO, 1993). It is up to the individual performing the analysis to assess the plot and determine which units should be combined for practical reasons of construction and economics (AASHTO, 1993).

In the case study, segments were made wherever the gradient changed signs. This was done because the speed profile was not recorded at constant distances or time as discussed above in Chapter 3.5.3. Table 3.22 summarises some of the segmentation results.

Table 3.22. Homogenous segment properties resulting from speed profiles.

Property Value

Number of segments 87

Average segment length 200.7 m Minimum segment length 5 m at 19 km/h Maximum segment length 1936 m at 58 km/h

The least number of segments was identified in Road Link 3. This link is also the shortest of the four links.