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Comparing Simulation Data with the HCM
Traffic simulation software has become an increasingly popular tool in transportation analysis. Simulation gives the user the ability to analyze the operation of complex systems under congested conditions and then analyze the impact of suggested improvements. Microsimulation is the modeling of individual vehicle movements on a second or subsecond basis for the purpose of assessing the traffic performance of highway and street systems. Microsimulation relies on random numbers to generate vehicles, select routing decisions, and determine behavior. Because of this variation, it is necessary to run the model several times with different random number seeds to obtain the desired accuracy. The warm-up period before the system reaches steady state should be excluded from the tabulated statistics. The output of a microsimulation model is different from that of the ‘’Highway Capacity Manual’’ (HCM). For example, most HCM procedures assume that the operation of one intersection is not adversely affected by the conditions of an adjacent roadway (with the exception of HCS 2000 Freeways). Long queues from one location interfering with another location would contradict this assumption. Measures of Effectiveness (MOEs) may be calculated or defined differently by simulation programs compared to how it is defined or calculated by the HCM. MOEs are the system performance statistics that categorizes the degree to which a particular alternative meets the project objectives.

Simulation Results
Microsimulation models usually produce two types of results: animation displays, and numerical output in text files. Animation can allow the analyst to quickly assess the performance, however it is limited to qualitative comparisons. The main indication of a problem in the model is the forming of persistent queues. For the numerical output, it is important to understand how the software has accumulated and summarized the results to prevent from incorrectly interpreting them. The following MOEs are most common when analyzing simulation models: •VMT (Vehicle Miles Traveled) is computed as a combination of the number of vehicles in the system and their distance they traveled. •VHT (Vehicle Hours of Travel) is computed as the product of the link volume and the link travel time, summed over all links. •Mean system speed is equal to VMT/VHT. •Total system delay is one of the most effective ways to evaluate different congestion relieving alternatives and it is usually the MOE that’s most irritable to the public. Delay can be calculated several ways. Some consider it to be only delay that is above free flow conditions. Others add in the base delay occurring at traffic control devices. Some even include acceleration and deceleration delay, and others include only stopped delay.

Comparison with HCM Delay and LOS
In the HCM, delay is used to estimate the Level of Service (LOS) for intersections. However, there are distinct differences between the way microsimulation programs and the HCM define delay. The HCM bases its delay on adjusted flow using mean control delay for the highest 15 minute period within the hour. The distinction between total delay and control delay is important. Control delay is when a signal control causes a group to slow down or stop. It’s important to look at the software’s documentation to understand how it calculates delay. In order to use microsimulation outputs to find LOS, the delay must be accumulated over 15 minute intervals and averaged over several runs with different random seeds. Because the HCM uses adjusted flow, another way to compare delay is divide the simulation input’s 15 minute peak volume by the peak hour factor (PHF) to increase the simulation’s volume.

Comparison with HCM Queues
HCM 2000 defines a queue as a line of vehicles, bicycles, or persons waiting to be served by the system in which the flow rate from the front of the queue determines the average speed within the queue. Slowly moving vehicles or people joining the rear of the queue are usually considered part of the queue. These definitions are somewhat relative and can be ambiguous. It’s important to note that in most microsimulation programs, the queue length cannot exceed the storage capacity for that turn-bay or lane. Overflows into the adjacent link or off the network are usually not accounted for. If this is the case, the best solution is to temporarily extend the network or storage area for the link to include the maximum queue length.