## 3 Model Description
### Variables  
The model requires input variables to run an experiment and output results that can later be analysed. The parameters that can be modified are listed in Table 1.  
The model consists of two entities: the vehicle agents and model environment. The vehicle parameters are:  
* The vehicle mass parameter, each vehicle can weigh up to 7500 kg; the model distributes vehicles arbitrarily across the environment with varying weights, from small cars to large goods vehicles (LGVs) to capture heterogeneity, every vehicle must have a mass of at least one such that the laws of gravity apply during the simulation experiment. Mass only becomes significant when collision severity is of importance; however, all collisions are considered in this study. * The top speed measure is between 30 and 45 mph, and is only applied to vehicles that do not adhere to speed limits (break speed limit rules), for example, vehicles that are driving on a 20 mph road can bypass the speed limit and drive at 45 mph which is more than double the speed limit. This measure is applied only if Speed Adherence is \(\geq\)1 (source [2]). * The ray-cast length parameter can be between 1 to 20. The variable assigns a distance between two vehicles in meters (source [56]). The environment specific parameters are:
* The number of vehicles in the model, \(N\); this can be between 1 and 500. * The speed adherence variable can be between \(0\leq x\leq N\). This assigns the proportion of vehicles that will not adhere to the speed limits (vehicles that break the local and fixed speed limits) applied to the road which they are driving on during simulation. * The urban road network consists of 1295 roads which vehicles drive on and 354 intersections which consist of right of way rules. The street network has been developed to depict a small urban town. The parameters mentioned above, once selected, are used to initialise the experiment (model-run) which lead to output variables. These variables observe data points every step of the simulation experiment. Table 2 describes the output variables that the model produces. The model outputs thirteen variables (refer to Table 2). The agent ID variable allows for a micro-level analysis of the agent behaviours during model execution at the street level, and this helps identify specific agents in the environment. The collisions variable is a cumulative number that increases each time the vehicle collides with another; this includes contact made between two or more vehicles on all road types and intersections. Top speed is the speed limit associated with the road that the vehicle is currently on, and the vehicle is trying to match the speed; however, in scenarios where some vehicles do not adhere to speed limits, the top speed for those vehicles would be a value between 30 and 45 mph ultimately breaking the speed limit. The current speed variable is the vehicle's speed at the current time of the simulation run. The distance of travel is in meters which tracks \begin{table}
\begin{tabular}{l l l} \hline \hline
**Entity** & **Parameter** & **Values** \\ \hline Vehicle & Mass & [1, 7500] (kg) \\  & Top Speed & [30, 45] (mph) \\  & Ray-cast Length & [1, 20] (m) \\ \hline Environment & N. Of Vehicles & [1, 500] \\  & Speed Adherence & [0, N] \\  & Roads & 1295 \\  & Intersections & 354 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Model entities and parameter values, where [X, Y] are a random uniform distribution of values (inclusive) [23]. the vehicle's distance from the starting position on the road network up until the current simulation step. The ray-cast length variable is the distance the vehicle can identify objects ahead, for example, other vehicles. Traction control is either 1 (on) or 0 (off). If the traction control is on, the vehicle has full traction capability such that each wheel can adapt to the surface; however, it is not utilised for this study as not all vehicles have access to traction control. The velocity magnitude is a scalar value demonstrating the rate of motion at a specific time.