§ 2.3.2. Site Generated Traffic

The following procedures are accepted practice in transportation planning and should be addressed in each report: trip generation, trip distribution and traffic assignment.

A.

Trip Generation.

Trip generation shall be based upon the proposed land use and density. A.M. peak, P.M. peak and total daily site-generated traffic must be calculated using an independent variable or determinant which has been confirmed by the transportation planner before the TIA is prepared. The applicant must identify and justify the applicability of the trip rates used. Gross square footage is the usually accepted determinant for office and gross leasable square footage is the usually accepted determinant for retail projects. The number of dwelling units is the most often accepted determinant for residential uses. A table of proposed land uses must also be included in each TIA report for review.

Trip generation rates shall be adopted from the references listed below. The second and third references may be used only when the information is not available in the first reference. If none of these sources contains appropriate trip rates for the proposed land use, other sources may be used at the discretion of the Director of the Watershed Protection and Development Review Department, if adequately documented.

1.

Trip Generation: An Informational Report , most recent edition, Institute of Transportation Engineers, 109 14th St., NW, Suite 300 W, Washington DC, 20005-3438.

2.

Travel Estimation Techniques for Urban Planning , NCHRP Report 365. Transportation Research Board, national Academy of Science, 500 Fifth St., NW, Washington DC.

3.

San Diego Traffic Generators , San Diego Association of Governments and CALTRANS District 11, January 1990.

Average weekday trip rates shall be used in estimating total daily trips generated unless otherwise indicated by staff in defining the scope of study. Weekend or other trip rates shall also be required if the peak hour does not occur on an average weekday. The average trip rate for peak hour of adjacent street traffic shall be used to estimate A.M./P.M. peak hour traffic entering and exiting the site. Guidelines in the Trip Generation manual shall be used for determining whether to use average trip rates or equations.

If the TIA is filed in conjunction with a site plan review, trip generation shall be based upon the uses and intensities identified on the site plan. If a site plan is not available, trip generation shall be based upon the maximum allowable density for the most intensive use. In such cases, the trip generation rates listed in Table 2-1 shall be applied, unless the applicant is willing to accept a density limitation as a condition of zoning or can document the fact that flood plains, watershed ordinance restrictions, compatibility standards or other factors limit the developable area of the site.

The rates provided in Table 2-1 are based on the maximum buildout allowed for each zoning category. The trip rates assume the most permissive land use, the maximum floor area ratio, the maximum building height and structure parking. For residential categories, the trips per acre are based on the number of units that are allowed for each zoning category.

Reductions for internal capture, pass-by traffic and transit usage should be discussed with the staff in defining the scope of study and must be supported by adequate documentation. No reductions in trip rates may be made for driveway turning movements unless it can be documented that certain trips will not use the driveway. Guidelines contained in the Trip Generation manual or other approved sources shall be used to document internal capture and pass-by trips.

B.

Trip Distribution.

Percentages for directional distribution of the site generated traffic must be well referenced. The basis for directional attraction shall largely rely on the following information:

Marketing Study

Subarea Transportation Study

City or State Travel Demand Estimation

If the above information is not available, a traffic study for determining directional attraction is required to address the area that is surrounded by the nearest arterial streets. The traffic study must be consistent with the procedures established for calculating trip distribution in Quick-Response Urban Travel Estimation Techniques and Transferable Parameters, NCHRP 187. , Transportation Research Board.

TABLE 2-1 TRIP GENERATION (BY ACREAGE)
Residential Development
	Maximum Units/Acre	Most intense Use (ITE Code)	Trip Rate/Unit	Trip Rate/Acre
LA	1.0	SF (210)	9.55	10
RR	1.0	SF (210)	9.55	10
SF-1	4.36	SF (210)	9.55	42
SF-2	7.58	SF (210)	9.55	72
SF-3	12.44	Duplex (NCHRP)	7.0	87
SF-4	12.1	SF (210)	9.55	116
SF-5	12.44	Duplex (NCHRP)	7.0	87
SF-6	12.44	Duplex (NCHRP)	7.0	87
MF-1	17.4	Low-rise (221)	6.6	115
MF-2	27.2	Low-rise (221)	6.6	180
MF-3	36.3	Low-rise (221)	6.6	240
MF-4	54.4	Hi-rise (222)	4.2	220
MF-5	54.4	Hi-rise (222)	4.2	220
MF-6	*	Hi-rise (222)	4.2	*
MH	9.68	MH (240)	4.8	46

 

TABLE 2-1 (Continued) TRIP GENERATION (BY ACREAGE)
Commercial and Industrial Development
Zoning	Permitted Intensity Far	Maximum s.f./Ac.	Most Intense Use (ITE Code)	Trip Rate/1000 s.f.	Trip Rate/Ac.
NO	.35	15.246	Gen off (710)	#	#
LO	.70	30,492	Med off (720)	34.17	1042
GO	1.0	43,560	Med off (720)	34.17	1488
CR	.25	10,890	*	*	*
LR	.5	21,780	Retail (820)	#	#
GR	1.0	43,560	Retail (820)	#	#
L	8.0	348,480	*	*	*
CBD	8.0	348,480	*	*	*
DMU	5.0	217,800	*	*	*
W/LO	.25	10,890	Gen off (710)	#	#
CS. CS-1	2.0	87,120	Retail (820)	#	#
CH	3.0	130,680	Retail (820)	#	#
IP	1.0	43,560	Med off (720)	34.17	1488
MI	1.0	43,560	Med off (720)	34.17	1488
LI	1.0	43,560	Retail (820)	#	#
R&D	.35	15,246	Med off (720)	34.17	521
* Trip rate to be determined on basis of proposed uses.
# Rate varies with acreage or building square footage.
Source: Institute of Transportation Engineers, Trip Generation , 5th Edition (1991). Transportation Research Board, Quick Response Urban Travel Estimation Techniques and Transferable Parameters (1978)

 

C.

Traffic Assignment.

This is the assignment of site generated traffic according to the percentages of distribution determined in the previous step. Traffic assignments shall be clearly illustrated with roadway and intersection geometry. The proposed roadway network shall conform to the Roadway Plan or other approved design by the Transportation Review Section.