Urban transport demand forecasts typically take into account many socio-economic and land use/planning data variables. However, parking is often over-looked. This, despite the fact that the demand for parking needs to be understood in order to enable the policy-makers to determine responses and inform the developers about what levels of parking they should try to provide for their customers.
Forecasts of demand for car parking are used in transport planning for a variety of purposes. Retailers wish to ensure that their customers have adequate parking space. Urban transport planners may, depending upon policy, wish to restrict car parking supply as one of a number of means to manage traffic levels. Car park operators, meanwhile, would wish to ensure that there is adequate demand for their facilities – both that they do not spend excessively on providing too many spaces and that they do not forego income by providing too few.
Whatever the stakeholders’ perspective, estimating current demand for parking can be fraught with difficulties, even before the challenges of predicting how that demand might grow in the future are faced. And within rapidly developing cities, forecast uncertainty is typically greater, thereby leading to even more challenges.
Economists define transport demand as a derived demand, i.e. one which arises as a consequence of demand for undertaking other economic activities (e.g. journeys to or from work, school, shopping or other activities). And parking demand is derived, in turn, from transport demand. There is also the question of whether measures of existing parking demand are, in fact, merely measures of existing parking supply, especially when many car parks are operating at or near capacity.
Challenges in estimating parking supply
- Even estimating existing parking supply can pose challenges. Inventories of parking supply are rarely maintained in rapidly developing cities (including most Chinese cities). But even when such inventories are kept, problems remain:
- Even where building lease conditions or other statutory approvals (e.g. traffic impact assessments) set out the approved number of parking spaces, this number may be exceeded by the developer.
- Vehicles may be parked on the side of the aisle, also leading to additional parking supply.
- Conversely, parking spaces may be diverted to loading/unloading and/or the storage of goods.
- Shop owners may informally (in some cases, illegally) reserve parking spaces outside their store for their own parking or loading/unloading usage.
- Vacant lots may be used as additional parking areas.
- Drivers may park on pedestrian pavements or into junction areas (shown in photographs).
- Parking capacity may also be affected by how closely together cars are parked in areas where individual bays are not indicated.
- Drivers dropping-off or picking-up passengers from public transport facilities or other buildings may also park (legally or illegally) for brief periods – which may prove difficult to counter, simply because of the brief stays. Yet, that can also have an adverse impact on the traffic flow in the area.
- Illegal on-street parking may take different patterns during night and day.
Measuring Informal Parking
In Hong Kong, in addition to a number of broad parking demand and supply studies, the government has commissioned parking inventory studies in order to attempt to measure the amount of informal parking, both at day and at night. One such study which the authors undertook was the “Parking Survey 2004 for Short Term Tenancy Sites, Unregistered Sites and Midnight Illegal On-Street Parking”, within which the overnight parking of goods vehicles was a particular focus.
Hong Kong is fortunate in that the government maintains a series of parking supply inventories. On the first Parking Demand Study, initial inventories were obtained from Buildings Department (inventories of parking spaces in new and existing buildings, from their Monthly Digest and from buildings plans), Architectural Services Department, Hong Kong Housing Authority, Transport Department and the on-street parking contractor (parking meter operator). However, owing to the issues stated above a comprehensive survey was still required in order to identify the actual supply of parking spaces, as well as to gauge the usage of parking spaces. Thus, frequent surveys of informal and illegal parking – as practised in Hong Kong – are strongly recommended.
In rapidly developing cities, older buildings are frequently torn down for redevelopment, posing a challenge to the upkeep of any parking inventories that are kept. This has been a particular challenge in China; the photograph on the next page illustrates the juxtaposition of old and new in Kunming, along with the examples of sites being cleared ready for new development. This reinforces the need for frequent and comprehensive independent surveys to be undertaken.
Complications in forecasting
This rapid pace of development witnessed in China is one of a number of factors which complicate forecasts of future parking demand. This is where the nature of demand for travel is derived from and in turn, the nature of parking demand gets based on that. This poses practical challenges. The population and economic growth and the impact of transport policies, together with a city’s urban form and the state of its transport networks (e.g. level of traffic congestion and provision of quality public transport services) can have a significant impact on the parking demand. We have noted over our combined experience that whilst parking demand growth might be benchmarked between cities, each city’s individual characteristics ultimately have to be taken into account. Indeed, a number of Chinese cities are now applying traffic demand management measures. Consequently, each city needs to be considered separately and each district within a city ought also to be considered in turn (depending upon the objectives of the study and resource availability).
Within Hong Kong, limited road space (2,050km of public roads) and parking space provisions have, over the years, served as a restraining measure on both the purchase and usage of cars. As such, Hong Kong has not had to rely on the Certificate of Entitlement adopted by Singapore. Hong Kong’s vehicle ownership is low at around 584,000 private vehicles for a population of just over seven million and an average per capita income of over US$31,000 p.a. (nominal 2008 data; annual 2008 income on a purchasing power parity basis was estimated at over US$44,000 per capita). Public transport mode share is around 90% which is one of the highest, if not the highest public transport mode share in the world. Restricted parking supply (properly enforced) can limit car ownership and hence, car usage and parking demand.
In China, car ownership has grown substantially in recent years. During the period 2002-2008, overall car ownership increased over 500% (over 35% per annum). However, there has been a divergence in growth rates between different regions: whilst Beijing averaged 27% p.a., some provinces averaged more than 40% p.a. over the same period. Such growth rates are straining urban transport networks and despite the provision of new car parks many are operating at or over capacity, as photographs with this article show.
In China cycling remains popular in almost all cities but almost inevitably many cyclists will aspire to car ownership. Cyclists can be interviewed regarding their travel habits and aspirations and whilst such surveys are useful, these alone are insufficient to gauge likely parking demand growth.
Where time series data exists on vehicle ownership and socio-economic variables, it is quite straightforward to set up econometric models to analyse historical statistical relationships and thus, to estimate future vehicle ownership with respect to economic and/or population growth. However, given S-curve relationships, wherein car ownership growth can first accelerate as a city’s income increases, then slow as demand becomes saturated, caution should be exercised with any long-range forecasts.
It should also be remembered that a given increase in car ownership does not necessarily give the same increase in car usage. At first sight, one might wish to replace car ownership data with traffic count data in order to estimate relationships of car usage (assuming city-wide traffic counts are not affected by changes to road networks). However, internationally, cities seeking to promote non-car transport have tended to concentrate upon peak hour travel; typically journeys to work or school. As such, families may still aspire to car ownership to provide mobility for social and leisure activities, yet continue to rely on public transport for their daily commute (assuming suitable “carrot and stick” measures are in place, i.e. providing attractive alternatives to car usage, whilst also actively dissuading car use). This means that other factors such as transport policy should still be considered.
Understanding the propensity of car usage
Rather than surveying just car users at a car park, it is important to interview people at their actual destinations (e.g. shops, workplace, etc) to ascertain not only the proportions of those who arrived there by car, public transport, bicycle or any other mode, but also to know the proportion of those who could have used a car and opted not to. Follow-up questions regarding the reason(s) why they did not use car should also be asked. This enables an understanding of the propensity for car owners to use a car. Yet even this provides but a snapshot of the situation. Even if data from a number of cities provides the opportunity for cross-sectional analysis, follow-up surveys are important. These can assess how such behaviour has changed over time, monitoring changes following the implementation of new transport policies or infrastructure (both roads and public transport services).
Experience also suggests that a given increase in economic growth would not necessarily produce the same increase in travel demand. People are likely to receive additional income per commute (wage rises) and to spend more per shopping trip. Population growth may therefore be more useful than economic growth when predicting travel growth, unless there is sizeable unemployment which is reckoned to be reduced as the economy grows.
Given all these uncertainties, it is therefore preferable to prepare a range of forecasts. And as stated above, the key recommendation is to perform follow-up surveys over time in order to monitor how parking demand has changed in the past, rather than simply relying on a single set of surveys.
Richard Di Bona, Director LLA Consultancy Ltd, Hong Kong
Wilkie Lam, Managing Director LLA Consultancy Ltd, Hong Kong