The Challenges Associated with Undertaking an Economic Appraisal of a Dynamic Managed Motorway Scheme
D Hardcastle, P Clarke, Mouchel; R Himlin, Highways Agency TAME, UK
This paper investigates the use of Initial and Full Intervention Investment Tool (IFRIIT) in assessing Dynamic HArd Shoulder (DHS) schemes.
Many motorways in the UK currently suffer from high levels of congestion throughout the day, which is a problem that is forecast to increase significantly over the coming years as demand for travel continues to grow. With limited availability of public funding, capital solutions to these problems are prohibitively expensive. Intelligent Transport System (ITS) solutions are a way of providing efficient innovative improvements aimed at maximising safety and reliability. The UK Highways Agency (HA) has developed a programme of schemes known as the Managed Motorways Programme aiming to deliver significant improvements to the operation of the motorway network using these ITS solutions.
One of the options incorporated within the Managed Motorways package of schemes is known as Dynamic Hard Shoulder (DHS) running. This entails the controlled use of the Hard Shoulder as a running lane (MM-HSR) during periods of high vehicle flow or incidents. It is recognised as an increasingly important tool in managing the motorway network. It provides a number of benefits compared to conventional road-widening; including cost effectiveness, speed of construction and an increased likelihood of implementation within the existing highway boundary.
These dynamic schemes do though cause significant complications for the transport planner, particularly related to issues with economic appraisal. The economic appraisal of a transport scheme is concerned with comparing journey times, vehicle operating costs etc with and without the scheme. This usually requires the use of a sophisticated transport model. In the situation where the hard shoulder is operating as a running lane there can be significant benefits for travellers brought about by the increased capacity. However, when the hard shoulder is in use, the maximum speed limit on the motorway is automatically reduced from 70mph to 60mph. The theory suggests that at the flow levels required to switch on the hard shoulder, traffic would already be travelling at below the maximum speed limit. However, if the hard shoulder is switched on prematurely (before this flow level is reached) this can generate dis-benefits to users brought about by the reduced speed limit.
This issue is further complicated because different sections of road can have different flow profiles. For example one section could be above the threshold for switch on and the next section could be below the switch on. In reality the operators are able to control the use of the hard shoulder on a link by link basis. In a transport model and in economic appraisal this is far more complicated. At any one time different links can be generating benefits and dis-benefits.
Another issue is how to annualise the benefits. Transport models usually operate for a finite number of time periods which are then factored up in the economic appraisal process to represent a whole year. This is relatively straightforward as we can easily calculate how many AM peak or PM peak hours there are in a year. However, how should benefits be annualised from the peak period models when the scheme is on for a certain proportion of the different peak periods? Also, do the average traffic levels predicted by the assignment model reflect the day to day variations of traffic that would be expected throughout the year?
This is a very complex process. The Highways Agency TAME group, in their role supporting major projects assessment, developed a tool known as the Initial and Full Intervention Investment Tool (IFRIIT). This is a spreadsheet based tool which aims to capture the dynamic affects of these types of schemes and interact with a variable demand/highway assignment model and economic appraisal software in order to undertake the economic assessment of a dynamic motorway scheme.
This paper investigates the use of IFRIIT in assessing DHS schemes. It analyses the theory behind its design, its inputs and outputs and the benefits and possible limitations of its approach. A case study of the M3M4 Managed Motorways Schemes is used to demonstrate the principles.
Association for European Transport