Trips generated by a proposed development, including vehicle and public transport trips, will typically add traffic to the existing transportation system surrounding the development. To the extent that new development generates new transportation impacts, it is reasonable to require sponsors of new development to contribute to the mitigation of the impacts.
This chapter establishes procedures for quantifying development impacts on the surrounding roadway and public transport systems. It is not always the case that these impacts need to be mitigated. This chapter therefore also defines thresholds for determining when these impacts are serious enough to warrant development of mitigation measures.
8.1. Quantifying Development-Related Transportation Impacts
Additional vehicle traffic generated by the proposed development will cause impacts to the roadway system, some of which may require mitigation. Similarly, development-generated demand for public transport services may impact the existing public transport system, and may require higher frequencies or new routes.
8.1.1. Traffic Impacts
The analysis of traffic impacts begins with a comparison of total traffic volumes and levels of service for a future with-development scenario (including all background growth plus the proposed development) to future baseline (including all background growth but without the proposed development) conditions on all intersections and links within the study area. Procedures for estimating future traffic volumes and levels of service have been provided in Chapters 4-7 of these guidelines.
Intersections to be analyzed may include access drives. For example, a large mixed retail/commercial building may come with a large underground parking garage whose peak-hour operation must be analyzed to ensure adequate performance and avoid operational problems such as the backing up of vehicles accessing the garage onto the adjacent roadway.
Quantification of development-related traffic impacts is achieved by carrying out the necessary analysis to complete the table shown in Table 8-1. For all relevant intersections and links in the study area, delay, V/C, speed, and levels of service are to be computed in a manner consistent with the methodologies and procedures described in Chapter 3 of these guidelines. Assumptions about lane geometry, signal phasing and timing, pedestrian volumes, percentage of trucks, and other characteristics of the facilities and the traffic using them should be the same in the future baseline and future with-development scenarios unless the proposed development description specifically changes the number of lanes or makes other changes to the features or utilization of a facility.
In the two columns in Table 8-1 labeled “Development-Related Traffic Impact” the proposed development’s traffic impacts are quantified, and it is determined whether these impacts are significant. The change in delay and/or V/C is determined by subtracting the future baseline value from future with-development value. Whether the impact is considered significant is indicated in the last column. Thresholds for determining if an impact is significant are provided in Section 8.2 of this chapter.
Table 8-1
Quantification of Development Related Traffic Impacts in Study Area
LOCATION EXISTING
Intersections Delay LOS Delay LOS Delay LOS Change in
Intersections Delay LOS Delay LOS Delay LOS Change in
Links Speed LOS Speed LOS Speed LOS Speed Significant?
Link #1
Link #2
…
Link #q
8.1.2. Other Transportation Impacts
Based on the specific characteristics of the proposed development and of the surrounding transportation system, the Roads Directorate may request additional issues to be analyzed besides the typical impacts discussed above. Examples of such issues are the operational adequacy of weave and merge/diverge sections, the performance of areas with high pedestrian volumes, etc.
Typically these additional impacts will be identified in the initial meeting and
In addition to the identification of traffic impacts described in this chapter, an analysis of multi-modal site/sector access and circulation is required. This analysis is described in Chapter 10 of these guidelines.
8.2. Thresholds for Determining Significant Transportation Impacts
To determine when development-related transportation impacts are serious enough to warrant development of mitigation measures, all transportation impacts identified in Table 8-1 are categorized as significant or insignificant. Significant transportation impacts are those quantitative impacts on the Level of Service or the average delay considered serious enough to warrant development of mitigation measures. The decision to classify an impact as significant or not is made using the concept of significance thresholds.
8.2.1. Approaches to Defining Significance Thresholds
There are two primary approaches to establishing significance thresholds. One approach is to establish LOS standards for the area, and then to consider any impact that causes a facility to become deficient (exceeding this standard) to be significant1. This deficiency approach essentially allows development to occur without mitigation until a deficiency is reached, and then require all development after that time to mitigate its impacts.
The second approach is to require developments to participate in the mitigation of impacts even before LOS standards are reached2. This typically requires more developments to implement mitigation measures, and tends to reduce the magnitude of mitigation to be implemented by any one development.
It has been decided to use the second approach in which the burden of mitigation is shared among a larger number of developments. This decision is based on the following:
• Strict adherence to the deficiency approach for mitigation could cause a development to implement large and costly mitigation measures even if its impact is small. For example, if an intersection has been fully improved and cannot be improved further with additional lanes or signal enhancements, an impact of as little as a few additional vehicles could force a development to build grade separations or build new roads in order to mitigate its impact.
• Strict adherence to the deficiency approach for mitigation in areas already congested (such as downtown) could conceivably
1 Examples of agencies that use the deficiency approach include Indiana Department of Transportation, Dubai Municipality, and Traffic Impact Studies Applicant’s Guide, and the San Diego Traffic Engineering Council.
2 An example of an agency using the shared participation approach is the Los Angeles Department of
result in a moratorium against any new development because LOS standards in the area have already been exceeded, and the cost of mitigating any new impact may be excessive.
• The deficiency approach could create a situation in which a very large development is built in an uncongested area with only a few low-cost mitigation measures, but using up all available transportation capacity in the area. A small development following behind it could then be required to implement very costly mitigation measures because all of the transportation facilities in the area have reached their LOS standards.
• The shared-participation approach allows for contributions of developments toward transportation infrastructure to be calculated based on their relative contributions to traffic growth in the region.
While recognizing that the private sector may contribute to the provision of transportation infrastructure, the Abu Dhabi Municipality and Town Planning Department will continue to take the lead in providing major new roadways and other transportation facilities that benefit the region. The shared-participation approach, which allows the development sector to contribute their fair-share to the program, may begin to be implemented in Abu Dhabi. Together, the public and private sectors can continue to be “ahead of the curve” in building a transportation system to support the future needs of the region.
8.2.2. Significance Thresholds for Abu Dhabi
Significance thresholds for development-related transportation impacts in Abu Dhabi that vary with the level-of-service of the facility are defined in Table 8-2. In line with the discussion in the previous section, these thresholds change as the level of service of the affected facility changes. If a facility operates at LOS E or F, then the threshold for categorizing an impact as significant is small. If the level of service of the facility is C or D, the significance threshold is larger.
Thresholds used in traffic impact analysis and other types of environmental impact analysis provide a consistent and rational approach to the assessment of impacts of developments or measures that reflects the underlying principles. It should however be borne in mind that the particular values and ranges for the LOS and the impacts that define these thresholds are based on professional judgment and experience, and as such are neither perfect nor the same for all agencies that use this approach to defining thresholds. Even within a particular agency they may be subject to review and modification over time.
The particular values listed here are based on the Traffic Impact Study Guidelines of the Los Angeles Department of Transportation and are felt to represent a useful
Table 8-2
Significant Thresholds for Development-Related Transportation Impacts
JUNCTIONS
1. Indiana Department of Transportation, Applicant’s Guide to Traffic Impact Studies, June 1993.
2. Dubai Municipality, Traffic Impact Studies Applicant’s Guide, 1997.
3. SANTEC/ITE,Guidelines for Traffic Impact Studies (TIS) in the San Diego Region, March 2000.
4. Los Angeles Department of Transportation (LADOT), Traffic Study Policies and Procedures, August 2003.