Traffic Impact Studies
Traffic impact studies evaluate the potential effects of proposed developments on the surrounding transportation network and identify improvements needed to maintain acceptable traffic operations. The study is typically required by local jurisdictions as part of the development approval process for projects that generate significant traffic. The study scope is determined through consultation with the reviewing agency and includes the study area boundaries, the analysis time periods, and the performance measures to be evaluated. The study area must encompass all intersections and roadway segments where the development traffic will exceed a threshold percentage of the existing traffic volume, typically 5 percent. The analysis periods are typically the morning and evening peak hours when traffic volumes are highest and the proposed development traffic will have the greatest impact on congestion.
The trip generation estimates the number of vehicle trips that will be produced by the proposed development based on the land use type and size. The Institute of Transportation Engineers Trip Generation Manual provides trip generation rates for different land uses based on empirical data from traffic studies at similar developments. The trip generation rates are expressed as trips per unit of development such as trips per dwelling unit for residential developments and trips per 1,000 square feet for commercial developments. The pass-by trips for commercial developments are trips that are already on the roadway network and stop at the development as part of their existing trip, reducing the new traffic added to the network. The trip distribution allocates the generated trips to the study area approach roads based on the existing travel patterns and the spatial distribution of the population served by the development.
The traffic assignment loads the development traffic onto the study area road network based on the trip distribution results. The addition of the development traffic to the existing traffic volumes provides the future traffic conditions for the analysis years. The level of service analysis evaluates the traffic operations at each study intersection under existing conditions, the projected background traffic without the development, and the projected total traffic with the development. The level of service is a qualitative rating of traffic conditions from A for free-flow conditions to F for congested conditions. The jurisdiction typically requires that intersections operate at LOS D or better after the development is built. If the development traffic causes the LOS to drop below the acceptable threshold, the developer is required to fund improvements that restore the LOS to the acceptable level. The improvements may include additional turn lanes, traffic signal modifications, roundabouts, or other capacity-enhancing measures.
Roundabout Design
Roundabouts are circular intersections that have gained widespread acceptance as a safer and more efficient alternative to traditional signalized intersections. The modern roundabout design requires entering vehicles to yield to circulating traffic, eliminating the high-speed angular collisions that occur at conventional intersections. The Federal Highway Administration reports that converting intersections from signals to roundabouts reduces injury crashes by 75 percent and total crashes by 35 percent. The reduction in crash severity is even more significant because the geometric design of roundabouts forces lower speeds through deflection, reducing the energy of collisions. The roundabout geometry includes the approach legs, the splitter islands, the circulatory roadway, and the central island. ite trip generation manual for traffic impact studies. modern roundabout geometric design for intersection safety. highway capacity manual level of service criteria for signalized intersections. The design vehicle determines the roundabout size, with the geometry designed to accommodate the largest vehicle that will use the intersection regularly.
The capacity of a roundabout is determined by the entry flow rate that can be accommodated given the circulating flow rate on the roundabout. The capacity analysis uses empirical models developed from observations of roundabout operations in different countries. The entry capacity decreases as the circulating flow increases, with the relationship depending on the number of entry lanes and the geometric characteristics of the roundabout. The critical gap is the minimum time gap in the circulating flow that an entering driver will accept to merge into the roundabout. The follow-up time is the time between successive vehicles entering from the same approach when there is a continuous gap in the circulating flow. The capacity models incorporate these parameters to estimate the entry capacity for each approach based on the geometric design and the circulating flow.
The pedestrian and bicycle accommodations at roundabouts are important design considerations that affect the safety and usability of the intersection for non-motorized users. Pedestrian crossings are located at least one car length back from the yield line to allow drivers to focus on the pedestrian crossing before entering the roundabout. The splitter islands at the approach legs provide pedestrian refuge and reduce the crossing distance to one lane at a time. The crosswalk markings and signage alert drivers to the presence of pedestrians. Bicycle accommodations at roundabouts can include on-road bicycle lanes that approach the roundabout, separated cycle tracks that bypass the roundabout, or shared lane markings that guide cyclists through the roundabout with vehicular traffic.
Highway Capacity and Level of Service
The Highway Capacity Manual provides the standard methodology for analyzing the capacity and level of service of highway facilities in the United States. The capacity of a roadway is the maximum hourly rate at which vehicles can reasonably be expected to traverse a point under prevailing roadway, traffic, and control conditions. The capacity of a basic freeway segment is approximately 2,250 passenger cars per hour per lane under ideal conditions. The actual capacity is reduced by heavy vehicles, lane width restrictions, lateral clearance limitations, and driver population characteristics. The level of service for freeway segments is determined by the density of vehicles per mile per lane, with LOS A at densities below 11 passenger cars per mile per lane and LOS F at densities above 45 passenger cars per mile per lane for free-flow speeds of 70 mph.
The analysis of signalized intersections in the HCM uses the control delay as the primary measure of effectiveness. The control delay is the additional time that vehicles experience due to the traffic signal, including deceleration delay, queue move-up time, stopped delay, and acceleration delay. The level of service criteria for signalized intersections are based on the average control delay per vehicle, with LOS A at delays less than 10 seconds and LOS F at delays greater than 80 seconds. The delay at signalized intersections is determined by the signal timing parameters, the traffic volumes on each approach, the number of lanes, and the progression quality along coordinated signal systems. The delay models in the HCM account for uniform delay from the signal cycle, incremental delay from random arrivals, and residual delay from oversaturated conditions.