# Enhancing Route Guidance with Integrated V2X Communication and Transportation Systems: A Review
## 3 Route Guidance Models  
Incorporating different real-time road traffic information in a traffic management center would help in generating more accurate estimates of travel time and efficiently guide the moving vehicles. Therefore, with the increased developments in connected vehicle technology, real-time vehicle RGSs have increasingly become crucial applications. Such systems are a kind of Traveler Information System (TIS) that supplies traffic information and travel recommendations to drivers to help them make better travel decisions. They aim to minimize the total travel time and travel injustice among individual travelers. It mainly considers the normal traffic operations and special events such as incidents, traffic jams, weather condition, etc. It recommends to drivers the optimal route from their current locations or origin(s) to their destination(s), which, in what follows, will be referred to as origin-destination (OD) pairs. To do so, TISs use mechanisms and processes to perform overlapping functions, such as determining network traffic flow and congestion and identifying optimum routes [16]. The assignment of vehicles by the RGS can target one of two assignment concepts: system optimal (SO) and user equilibrium (UE). System optimal guidance aims at improving travel conditions for the entire network. This means that some drivers might be routed to sub-optimal routes to balance and enhance overall network flow [17; 18; 19; 20]. User equilibrium guidance aims at optimizing the route of individual drivers without considering the network performance. When all travelers between the same OD pairs have the actual and lowest cost, user equilibrium is attained. Additionally, unilateral route changes cannot be used to lower any traveler's transportation costs [21; 22; 23; 24; 25]. Route guidance can be descriptive guidance, which provides information on traffic and road conditions with no routing advice, for instance, a variable message sign (VMS) that provides drivers with current estimates of driving time to an upcoming highway intersection [26; 27], or prescriptive guidance, which offers routing advice but no information on traffic network conditions, such as an online mapping and driving direction system [21]. In some implementations, descriptive and prescriptive system elements can be integrated [28]. RGS can be categorized based on when the guidance is preformed: either pre-trip or on-route [21; 29]. There are desired properties and criteria for traffic  
Figure 1: PRISMA flow diagram results. assignment models within large-scale strategic transportation systems [30; 31; 32]. A few important indicators for assessing the success of an RGS will be discussed in more detail in what follows. * Tractability: The theory underlying each component or sub-model can be used to validate the computations of the assignment model [32; 33]. * Accuracy under congested conditions: The extent to which the model accounts for the impact of congestion on flow metering, spillback, and route choice [32; 33]. * Accountability: The degree to which different model components can be separated and examined independently [32; 33]. * Robustness: The extent to which a model's results remain unaffected by random variables and how well the model converges to a clearly defined and stable state [32; 33]. * Efficiency of computation: The total running time and memory requirements that can be accommodated for calibration and use in large-scale models. * Input requirements: The extent to which input criteria are available with acceptable levels of uncertainty for scenarios in the distant future [32; 33]. * Applicability: The extent to which the model is relevant to all vehicle classes and suitable for both urban and highway roads [32; 33]. The relative gap (RGP) of travel time is a common metric for convergence that allows comparisons between the performance of different iterations of routing strategies or algorithms for various traffic assignment models.