Multimodal Operations Analysis / Traffic Operations Analysis
When changes are made to the transportation system, like adding lanes, changing lane allocation, adjusting traffic signal timing, or even replacing an intersection with a roundabout, it is important to understand how these changes will impact all road users: drivers, pedestrians, and bicyclists.
This evaluation is called traffic operations analysis or, more appropriately, multimodal operations analysis, and it is a critical step in the success of any project. The choice among alternatives depends on the outputs of this step, therefore, it requires meticulous attention to detail and thorough quality control.
The Highway Capacity Manual (HCM) provides methodologies to determine the Level of Service (LOS) for different road users, as well as predict queue lengths. The current version of the HCM is the 7th edition. For accurate application, a traffic engineer must have in-depth knowledge of these methodologies, including their input parameters and how to interpret their results. It's important to remember that HCM methodologies are generally designed for analyzing unsaturated facilities in isolation, and each methodology has specific limitations. The results from HCM are valid only within these limitations.
For conditions that fall outside the scope of HCM methodologies, or for more complex operational analysis problems, micro-simulation is utilized. Applying micro-simulation software effectively demands a specialized skillset that requires years of study and experience. Modelers must be proficient in understanding the driving behavior models embedded within the software and how different objects interact internally. This knowledge is crucial for accurately coding the network and calibrating the model to reflect real-world conditions.
For insights into comparing micro-simulation results with the HCM, please refer to our two articles below. The first article focuses on LOS criteria for different types of facilities and the second one focuses on queue length at intersections.
Multimodal Intersection Analysis
To analyze intersections (e.g., roundabouts) using HCM 7th methodologies, geometric data and other relevant details are collected via field visits and online sources.
Historical crash data at the intersection is also reviewed to identify common contributing factors that can inform future design improvements.
Typically, traffic counts for vehicles, bicycles, and pedestrians are collected. This usually requires a minimum of 3 to 4 hours each in the morning and afternoon, with data recorded in 15-minute intervals. This approach helps to capture the distribution of traffic demand during peak periods. While new data collection technologies can lead to higher quality data, all collected data still requires thorough error checking. From this collected data, the morning and afternoon peak hours are determined for the HCM-based analysis.
Traffic growth is projected based on various sources, such as historical growth patterns, planned nearby developments or land use changes, and forecasted volumes from travel demand models.
For signalized intersections or new signalization, traffic signal data for the intersection and surrounding coordinated intersections should be requested from the signal operator. This data is then cross-referenced with field observations. Additionally, parameters like yellow, red clearance, pedestrian clearance, and left-turn requirements should also be calculated based on the Manual on Uniform Traffic Control Devices (MUTCD) and common practice. This allows for comparison with provided values and establishes a basis for calculating these parameters for proposed alternatives.
It's important to note that the HCM 7th methodology for roundabouts primarily takes into account capacity and conflicting flows at entry points. Weaving on the circulating lanes is not precisely accounted for. As a result, the HCM 7th tends to overestimate the roundabout's capacity and LOS.
Should the study area involve a roundabout, or if the intersection is particularly complex or oversaturated, or there's a need to study pedestrians in detail, a micro-simulation analysis is conducted as the next step.
Multimodal Corridor Analysis
The urban street segments methodology of HCM 7th is designed to assess the frictional impact of elements like driveways and other similar features on a continuous street segment, typically up to 2 miles long, situated between two intersections.
This methodology allows for the calculation of the LOS for vehicles, transit, pedestrians, and cyclists along the corridor. When a full corridor, including its intersections, needs to be analyzed comprehensively, a micro-simulation analysis is conducted.
Two-Lane Highway Analysis
The two-lane highway methodology of HCM 7th analyzes continuous segments of two-way corridors, with one lane in each direction, that are longer than 2 miles. In HCM 7th, the methodology has been considerably changed compared to previous versions. In this version, the LOS for vehicles is determined based on the follower density (followers/mi/ln), with slightly different ranges used for higher-speed and lower-speed highways.
In summary, this method first involves breaking the corridor into continuous uniform segments. Next, the vertical alignment class is determined based on the grade (slope) and segment length. Subsequently, the free-flow speed is measured or calculated. Average speed is then calculated and adjusted for horizontal curves. The percentage of followers at 25% of capacity and 100% of capacity is computed based on vertical alignment, length, free-flow speed, percentage of heavy vehicles, and opposing flow rate (for passing zones). This creates an exponential curve of percent followers versus flow rate for the segment. Follower density is then estimated and subsequently adjusted for passing lanes. The LOS is then determined.
Freeway and Multi-Lane Highway Analysis
The freeway segments methodologies of HCM 7th are developed to analyze basic, weaving, and merge/diverge segments in isolation. While Chapter 10 combines these to assess connecting freeway segments and account for spillbacks, and Chapter 38 extends this to freeway-arterial connections with spillback considerations, a micro-simulation model typically offers a more reliable approach for such complex studies.
Simulating freeways, their service roads, and interchanges requires a special set of skills in macroscopic and mesoscopic modeling, and dynamic traffic assignment (DTA), in order to estimate dynamic traffic demand. Accounting for changes of demand over time is necessitated by the large size of such models, resulting in long travel times, and also the need to simulate multiple hours of the day for each peak period to cover formation and dissipation of congestion. After the demand is estimated, it's also recommended that DTA be utilized in the micro-simulation model to more realistically model vehicle behavior.
