Challenge

Traditionally, formulating a road upgrade strategy has been the product of several disparate studies and investigations of a corridor. These studies often work in isolation and do not account for the complex interactions between disciplines that are often mistakenly viewed as being partly or wholly independent of each other.

SMEC South Africa was appointed by Trans African Concessions (TRAC) to design the upgrade of N4-6Y (MDC Section 6N – colloquially known as Schoemanskloof). This 62-kilometre stretch of road, located in Mpumalanga, needed an upgrade that adhered to stringent contractual requirements, notably maintaining a Level of Service (LOS) of no worse than LOS D by 2028.

The existing conditions consisted of a two-lane cross-section (one lane per direction). Opportunities to overtake were severely limited by the undulating vertical alignment and winding horizontal alignment.

The challenge for the team was to integrate the various disciplines and workflows and to develop an innovative approach to unify all the engineering factors and considerations into a single platform so that nothing is overlooked.

Solution

To achieve a safe and acceptable driver experience with only passing lanes, SMEC had to take a host of constraints and considerations into account to develop a financially feasible strategy. Had conventional approaches been followed, it is likely that a full four-lane road upgrade would have been recommended at approximately four times the estimated construction cost using this holistic assessment approach.

The Holistic Upgrade Strategy is an innovative, all-encompassing approach in planning the upgrade of two-lane rural highways in terms of optimising performance in relation to upgrade cost.

The following key elements were considered and presented in this holistic road assessment map:

• Horizontal Alignment: Identifying below-standard horizontal curves and their impact on safety and LOS.
• Vertical Alignment: Analysing vertical geometry, steep grades, and compound grades to optimise climbing or passing lane placement.
• Speed Profile: Recognising the speed limitations of heavy vehicles on steep grades, leading to queues and accidents.
• Vissim Microsimulation: Utilising a microsimulation model to assess LOS in terms of Average Travel Speed.
• Safety: Identifying accident hotspots and incorporating them into passing lane placement.
• Strip Map: Creating a comprehensive road layout strip map that integrates road components, structures, environmental factors, intersections, and accident data.

For the proposed upgrade strategy, the abovementioned road components were analysed and presented in a holistic road assessment map. The map graphically overlays each element in easy-to-understand terms, whilst highlighting any variables that might be impacted by physical constraints or cost effectiveness. The road layout strip map was drawn up based on the topographical survey and various other data sources, including:

• River bridges and major culverts
• Intersections and major accesses
• Locations of borrow pits as sources for materials. Given the scarcity of construction materials, it was found to be essential to include these sites for the estimation of overhaul costs
• No overtaking zones
• Existing passing lanes
• Proposed new passing lanes or passing lane extensions
• Significant environmental/ heritage/ social features to be avoided