Development Trip Generation

The estimation of the traffic generated by the development is a crucial element of the Traffic Impact Study process. This will usually include the use of trip generation rates, as discussed in detail in this chapter. It is the responsibility of the applicant to develop peak-hour trip generation estimates for the proposed development and to fully document the assumptions and methods used in developing those trip generation estimates. The trip generation estimates, with full supporting documentation, are to be submitted to the Roads Directorate for review and approval.

In certain rare cases, the proposed development to be studied may be so large as to warrant the use of the Abu Dhabi Travel Demand Forecasting model for estimating trip generation. The possibility of using the model instead of the trip generation rate approach outlined below should be explored with the Abu Dhabi Municipality.

6.2.1. Peak Hours

The choice of appropriate peak hours is crucial to the accurate assessment of the impacts. As described in Chapter 3, the specific peak hours to be analyzed will be determined in the initial meeting and documented in the Preliminary Plan. The critical traffic time period for a given development is directly related to the peaking characteristics both of the development-generated traffic and of the adjacent road network. Generally, it has been found that the following peak hours are likely to cover the peak hours for many locations in Abu Dhabi:

• Morning peak hour (7:00-8:00 AM);

• Noon peak hour (11:00 AM - 12:00 Noon);

• Afternoon peak hour (2:00-3:00 PM); or

• Evening peak hour (7:00-8:00 PM).

These should cover typical locations in the CBD. However, local area characteristics may result in other peaks for the adjacent road network, and traffic counts. The peaking characteristics of the adjacent road network can be determined through analysis of traffic count data, which, if not available, may need to be collected for the purposes of the study.

The peaking characteristics of the development result from the trip-making characteristics of the land uses within the development site, and may differ from that of the adjacent road network. For the design and evaluation of access drives to the development, the peak periods for development traffic should obviously be used.

The periods that result in the highest cumulative directional traffic demands should be used to assess the impact of site traffic on the adjacent road network and to define the roadway configuration and traffic control measures changes needed in the study area.

The consultant should explain his choice of peak hours for both the development and the adjacent road network.

6.2.2. Trip Generation Rates

The most frequently used approach to estimating trip generation is to apply trip generation rates to the number of square meters and/or number of residential units of individual land uses. The trip generation rates typically represent the average number of peak-hour vehicle arrivals and departures per unit of development, established from surveys or other procedures.

Trip generation rates estimated for the city of Abu Dhabi do not currently exist. The Roads Directorate will provide samples of approved trip generation estimation procedures from completed Traffic Impact Studies, to assist the applicant in estimating trip generation for the specific development being analyzed. Until local Abu Dhabi data becomes more common or an Abu Dhabi trip rate manual is developed, alternative sources for trip rates, including the Dubai and ITE trip generation manuals (1,2), may be considered. It may be necessary to apply adjustments as appropriate to the trip rates obtained from these sources to obtain suitable peak hour trip generation rates for developments in Abu Dhabi.

If in the process of developing trip generation rates the applicant carries out trip generation surveys for existing land uses, whether in Abu Dhabi or elsewhere, then the results of those surveys are to be included in the documentation, and submitted both in printed and electronic format. The Roads Directorate will archive these surveys results. As time progresses, the Roads Directorate will make the growing database of such survey results available to others as appropriate to assist them in developing trip generation estimates.

Some important adjustments to trip rates are discussed below. These may need to be performed as a result of the nature of the development and the surrounding road network. Other adjustments may be proposed or used by the consultant.

6.2.3. Primary vs. Linked Trips

Trip generation rates such as those reported in ITE and Dubai are derived from actual measurements of traffic generated by individual sites, representing vehicles entering and exiting at the development’s driveways. Therefore, these volumes are appropriate for determining the total traffic to be accommodated by the development’s driveways.

There are instances, however, when the total number of trips generated by a development is different from the amount of new traffic added to the surrounding street system. For example, retail-oriented developments such as shopping centers, restaurants, banks, service stations, and convenience markets are often located adjacent to busy streets in order to attract the motorists already on the street. These developments attract a portion of their trips from traffic passing the site on the way from an origin to an ultimate destination different from the new development. Trips attracted in this way clearly do not represent new traffic on the surrounding road network. Such pass-by or undiverted linked trips are defined as trips attracted to a particular development from the traffic “passing by” on the adjacent roadway (3,4).

These trips are not newly generated trips made for the purpose of visiting the new development. For such uses, a reduction in the effective trip generation may be warranted when estimating the amount of new traffic on the adjacent road network.

A similar type of trips attracted by the proposed development but not newly generated is that of diverted linked trips. These trips divert from their original route to travel to the development site, thereby adding traffic volume to the roadway links that connect the original route to the development. On the other hand, trips that are made solely for the purpose of visiting the development are named primary trips. The different trip types are illustrated in Figure 6-1.

The percentage of pass-by and diverted linked trips varies by the type and size of land use, time of day, day of the week, geographic location of the site, and nature of the roadway network serving the site. A reduction in trip generation for the two types of linked trips may hence be applied.

Figure 6-1:

Trip Types (Source: Institute of Transportation Engineers, Trip Generation Handbook, 2001)

6.2.4. Internal Capture

especially when these within-site trips can be made on foot. As a result, the total generation of vehicle trips entering and exiting the multi-use site may be less than the sum of the individual, discrete trips generated by each land use. A common example is a multi-use development that contains offices as well as a shopping/service area.

Some of the trips made by office workers to shops, banks, or restaurants are accomplished on site. These trips are defined as internal to (i.e. “captured” within) the multi-use site.

A reduction in trip generation rates to account for internal capture may therefore be needed in some cases, when applying the trips sequentially to the different land uses comprising the development. (Trip rates calibrated for multi-use developments do obviously not have to be modified). This should be appropriately justified and documented.

6.2.5. Mode Split Variations

Whenever the mode split conditions for the traffic impact study are expected to differ significantly from those under which the trip rates were calculated (in a manner that affects vehicle trips), then appropriate adjustments should be made. For example, ITE trip rates are calculated for sites at which the private car is the almost exclusive mode used to access the site. To use ITE areas to estimate trip generation for a development located in an area well served by public transit would require some adjustment to account for the reduced proportion of vehicles traveling to the site that results from the higher public transportation mode share.

Smaller mode share for automobiles will reduce vehicle trips if auto trips are replaced by pedestrian or bus trips. In Abu Dhabi, taxis have a larger mode share than typical in North America. However, a taxi trip is still a vehicle trip, and may not reduce total trip generation, only parking demands.

A modification to trip generation for mode-split variations may therefore be applied in some cases. This modification should be appropriately justified and documented.

6.2.6. Tables of Existing and Future Development Trip Generation

The results of the trip generation estimation procedures are to be documented in the Traffic Impact Study report. Tables of existing conditions on the development site and future conditions after the development is built out and fully occupied are to be developed. Each phase of development is to be included separately in the tables. The tables should include all trips within the development site. If the proposed development is located on a site that contains existing trip-generating land uses, then the tables of existing and future trip generation can be used to calculate the net increase in trips due to the proposed development.

6.2.7. Information On Available Trip Generation Resources

Since the objective is to find the number of trips generated by the yet-to-be-completed development undergoing the Traffic Impact Study, some form of estimation must be used. A discussion of both the ITE and Dubai trip rates as sources of trip generation

data that may be used in developing trip generation estimates for proposed developments in Abu Dhabi is provided in Appendix A.