Design speed for new high-speed lines

There are examples where the design speed of new high-speed lines has been based more on performance of present technology than on optimisation of the cost-benefit outcome considering future needs. A model for optimising the design speed, or intended dimensional speed, for new high-speed lines to attain more cost-efficient planning is presented in this paper. This model, HSLdim, includes a travel demand sub-model where the dependent variable is commercial speed, and design speed dependent infrastructure cost and train operating cost sub-models, adapted for fast optimisation. Some aspects of a proposed high-speed network in Sweden have been analysed. The average speed should be as high as possible to maximise benefits. The optimal design speed for the Swedish case was found to be 325-350 km/h depending on the share of ballastless track, which has a significant influence on the construction costs. This is in line with international experience but somewhat higher than the planned design speed of 320 km/h. To summarise, the paper gives an example that analysis could increase the socio-economic output by optimising the design speed in a CBA to take future needs into consideration in new high-speed rail projects.