Optimum Road Surface Design

1. Optimum Road Surface Design John Hine International Division TRL Limited

2. Past Solutions: Prior to road investment models the common rule of thumb was used: • 0- 50 ADT Earth road • 50- 200 ADT Gravel road • > 200 ADT Bitumen surfaced road

3. Modelling to Give an Optimum Solution • Highway cost models combine: Road deterioration relationships Works effects Construction & maintenance costs Vehicle operating cost relationships Vehicle, fuel, labour and tyre prices

4. What is the optimum traffic level to switch using modelling? New Research results:- • Zimbabwe 95 : 40 ADT to seal gravel road ! • Zimbabwe 99 : 70 ADT to seal gravel road ! Are the results transferable ? - No Zimbabwe has a dry climate, poor (surface) gravels and low construction costs

5. The costs of upgrading roadsCost per km

6. Conclusions 1. • There is a huge range of costs for paved roads; the costs of the bitumen surface is only a small part of the total • It is clearly dangerous to automatically transfer Zimbabwe findings to other countries • Engineers are very risk adverse and often take every opportunity to spend money to “protect” their investment, hence paved roads can end up being very expensive

7. Conclusions 2. • A lack of maintenance is no argument for paved roads: all roads need maintenance - otherwise you are just throwing away money! • Most countries only regravel about 2% of their “gravel” network each year rather than the requirement of say 17% per year • The economic window for gravel may be small (between earth and bitumen) but it depends on climate and available materials • We need to educate engineers in how to take risks and design very low cost paved solutions