| Vehicles equipped with Intelligent Speed Adaptation, or ISA, generally travel slower than vehicles that are not equipped with ISA. This reduces the crash risk of the ISA equipped vehicle. Another benefit of introducing ISA equipped vehicles into the fleet is that by travelling slower, they cause other vehicles in the network to also travel slower.
There may be a ‘critical mass’ of ISA penetration at which the benefits of ISA accrue rapidly for the entire vehicle network, regardless of whether or not they are equipped with ISA. This report seeks to determine if such a critical mass exists.
Micro-simulation software (PARAMICS) was used to simulate a simple traffic scenario. This scenario involved traffic moving at typical urban speed limits along a straight road. The proportion of vehicles with ISA was increased from zero to 100%, at increments of 5%. The network was modified to include different speed limits, traffic volumes, signal densities and multiple lanes.
The results of the simulations suggest that the adoption of ISA does influence the speeds of vehicles both with and without ISA. The benefits of ISA accrued most rapidly for lower ISA penetration. The mean speeds of all vehicles continued to decrease as the proportion of vehicles with ISA was increased to 100%, but at a lower rate. For a basic scenario, at around 30 to 40% ISA penetration, the speed distribution of all vehicles was close to the speed distribution of ISA equipped vehicles.
The speed limit of the urban scenario did not influence the effectiveness of ISA in reducing all vehicle speeds. However, increased traffic volume increased the effectiveness of ISA, and ISA was more effective on a single lane road than a two lane road. The influence of traffic signal density was not straightforward and was confounded by the formation and disaggregation of platoons when caught at a red light.
A higher speed rural road was also simulated, with and without the presence of an overtaking lane. For the rural road, a similar effect on vehicle speeds was observed with increased free speed of ISA. Again, traffic volume increased the effectiveness of ISA at reducing all vehicle speeds. The introduction of an overtaking lane decreased the effectiveness of ISA, by allowing faster travelling non-ISA vehicles to overtake.
The introduction of ISA equipped vehicles into the fleet has benefits for the occupants of those vehicles, but also for the fleet as a whole. No optimal ISA penetration rate was observed however the most rapid reductions in average travelling speeds occurred in the first 40% of ISA penetration.
The modelling revealed the complexity of the relationship between ISA and traffic variables such as average speed. This makes the development of a universal theoretical model difficult to establish and simulations of real world networks would be necessary to account for the range of interactions that occur. This was considered outside of the scope of this current exploratory project. | 