ITE Journal April 2018 - 42

The standard approach for parking ratios in zoning regulations
is to use the 85th percentile of the peak ratios given by ITE's Parking
Generation, i.e. the ratio that is exceeded by 15 percent of the survey
samples that underlie the ITE parking data. If a planner, designer,
or municipality were to use the average of all peak ratios surveyed,
there is a 50 percent likelihood that there would not be enough
parking. However, in a mixed-use development with perhaps five
or 10 different uses, it is not likely that all uses represent worst-case
parking conditions. Hence, the safety margin will be greater, when
more uses share the same parking supply and the 85th percentile
ratios are used for the calculation.

Conditions Necessary for Shared Parking
A successful shared parking scheme requires that the shared
parking facility be in a convenient location for all users.
Convenience depends to some degree on walking distance, but the
quality of the walk to and from the parking space is probably as
important as the distance. The other critical requirement is that no
spaces be assigned to individual users. Any assigned or reserved
spaces need to be taken out of the shared parking pool.
Can a residential development have a combination of assigned
spaces and shared spaces, where each unit gets one assigned space
and those units that need more than one space get the additional
space in a shared area? This combination is theoretically possible;
however, in practice we don't know how two-car households behave
when they have one car in the assigned space and one car in the
shared area, and one car is used on a daily basis for commuting
and the other car is only used on an occasional basis (a frequent
situation for two-car households). It is likely that the car that moves
on a daily basis will be parked in the assigned space, and the car
that moves less often will be in the shared area, a condition that
significantly negates the benefits of shared parking.

Frequent Concerns and Questions
When shared parking projects are presented to planning boards or
elected officials, a frequent question or concern is "what happens
if one day I have an emergency and have to come home at noon?"
These concerns are addressed by maintaining a margin of vacant
spaces so that there is always flexibility to accommodate such
events or daily fluctuations. If the underlying occupancy data
were collected in a large residential development of more than
100 units, the percent occupancies throughout the day probably
include users that are in those special conditions-being home sick,
having an infrequent visitor, cleaning person, or caretaker, etc. Or
"what happens on Black Friday when some people don't work and
are likely to stay home"? In a case like this, if the sharing occurs
between an office and a residential project, the residential parkers
staying home (if they really stay home instead of going shopping
or traveling out of town) are offset by the reduced parking load
42

Ap r i l 2018

i te j o urn al

from the office users on that day. If parking is shared between retail
uses and residential uses, and the retail use is affected by holiday
shopping, the shared parking analysis needs to take into consideration the December parking demands of the retail uses.
In transit oriented developments (TOD) there is often a desire
to share parking between residential users and rail commuters.
Shared parking in TOD situations needs to take into consideration
several special factors: 1) car ownership of the residents will be
lower; 2) the percentage of cars staying in the garage during regular
working hours tends to be higher (but there are still some residents
that commute by car); and 3) do the residents leave early enough
to make room for the rail commuters? The response to the last
question depends largely on the location of the station in relation to
the downtown area. If the station is 90 minutes from the downtown
area, the percent of residential spaces freeing up early enough to
accommodate the rail commuters is probably too small to justify
a shared parking program. But if the station is half an hour away,
the timing is such that the rail commuters arrive at a time when
sufficient residents have left the garage. The graph in Figure 2
shows how up to 137 rail commuters could be accommodated in a
garage of a residential development adjacent to a rail station that is
28 minutes from downtown Manhattan. At 3 a.m. the residential
development had 83 vacant spaces out of a total of 306 spaces. In
and out counts determined the garage occupancies at various hours
of the day. At 10 a.m. there were still 62 percent of the residential
cars in the garage. That provided sufficient vacant spaces to allow
the owner to sell parking permits to 137 rail commuters and still
maintain a 10 percent reserve. The arrival and departure pattern
of the rail commuters was based on hourly counts at an adjacent
commuter garage. In this case, the oversupply of parking at 3 a.m.
was part of the reason allowing that number of commuters to park.
Without oversupply of parking there would have been room for
about 50 commuter cars. In conclusion, shared parking between
residents and transit commuters in TOD projects is generally more
limited but is feasible if the TOD is located relatively close to the
downtown area.
Sometimes there is a question whether there should be signage
in a shared parking facility indicating that, for example, the office
employees should vacate their spaces by 6 p.m. to make room for
the returning residential parkers. This is generally not required
and not recommended, mainly because not all residents return at 6
p.m. The shared parking analysis should take these shoulder times
in consideration. Occupancy surveys at three residential projects
located in mixed-use/urban environments in the New York/New
Jersey Metropolitan area have shown the following presence of
residential cars in the early morning and evening hours:
6 AM 95%; 7 AM 83%; 8 AM 63%; 9 AM 53%
5 PM 55%; 6 PM 62%; 7 PM 69%; 8 PM 74%



Table of Contents for the Digital Edition of ITE Journal April 2018

President’s Message
Director’s Message
People in the Profession
ITE News
Technical Programs Division Spotlight
Get Ready for Minneapolis!
Ethics Column: Responsible Engineer
Member to Member: Sammy Xiaoming Chen, Ph.D., P.E., PTOE, PTP
Where in the World?
Calendar
Industry News
New Products
Love My Hood: Kitchener’s Resident-Led Traffic Calming Program Advances Safety and Placemaking
Placemaking through a Traffic Signal Box Art Project: Effect of Artist Wrap to Cabinet Temperature
Shared Parking: Effective and Simple
Relationship between Geometric Elements and Wrong-Way Crashes at Partial Cloverleaf Interchange Terminals
Professional Services Directory
ITE Journal April 2018 - 1
ITE Journal April 2018 - 2
ITE Journal April 2018 - 3
ITE Journal April 2018 - President’s Message
ITE Journal April 2018 - 5
ITE Journal April 2018 - Director’s Message
ITE Journal April 2018 - 7
ITE Journal April 2018 - People in the Profession
ITE Journal April 2018 - 9
ITE Journal April 2018 - 10
ITE Journal April 2018 - 11
ITE Journal April 2018 - ITE News
ITE Journal April 2018 - 13
ITE Journal April 2018 - Technical Programs Division Spotlight
ITE Journal April 2018 - 15
ITE Journal April 2018 - Get Ready for Minneapolis!
ITE Journal April 2018 - 17
ITE Journal April 2018 - Ethics Column: Responsible Engineer
ITE Journal April 2018 - 19
ITE Journal April 2018 - 20
ITE Journal April 2018 - Member to Member: Sammy Xiaoming Chen, Ph.D., P.E., PTOE, PTP
ITE Journal April 2018 - 22
ITE Journal April 2018 - Calendar
ITE Journal April 2018 - New Products
ITE Journal April 2018 - 25
ITE Journal April 2018 - 26
ITE Journal April 2018 - Love My Hood: Kitchener’s Resident-Led Traffic Calming Program Advances Safety and Placemaking
ITE Journal April 2018 - 28
ITE Journal April 2018 - 29
ITE Journal April 2018 - 30
ITE Journal April 2018 - 31
ITE Journal April 2018 - Placemaking through a Traffic Signal Box Art Project: Effect of Artist Wrap to Cabinet Temperature
ITE Journal April 2018 - 33
ITE Journal April 2018 - 34
ITE Journal April 2018 - 35
ITE Journal April 2018 - 36
ITE Journal April 2018 - 37
ITE Journal April 2018 - 38
ITE Journal April 2018 - Shared Parking: Effective and Simple
ITE Journal April 2018 - 40
ITE Journal April 2018 - 41
ITE Journal April 2018 - 42
ITE Journal April 2018 - 43
ITE Journal April 2018 - 44
ITE Journal April 2018 - Relationship between Geometric Elements and Wrong-Way Crashes at Partial Cloverleaf Interchange Terminals
ITE Journal April 2018 - 46
ITE Journal April 2018 - 47
ITE Journal April 2018 - 48
ITE Journal April 2018 - 49
ITE Journal April 2018 - Professional Services Directory
ITE Journal April 2018 - 51
ITE Journal April 2018 - 52
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