Author: Gordon Bell
I ask this question because I have seen in recent years various references to this subject in the literature. Generally, the quote is along the lines of “On average, 30% of traffic on urban roads is caused by people searching for a parking space”.
Most such statements are not qualified by relating them to the type of road under study - are they all roads, congested roads, downtown roads or as suggested in the above quote urban roads? Do they relate to all time periods or just peak periods? Do they relate to roads where parking is free; where the demand for parking is high compared with the supply; where parking enforcement is weak or strong? Each situation would surely produce different results.
My aim is to look into the background of this subject and, hopefully, to draw attention to the mis-use and, dare I say it, the abuse of such statistics.
First let me quote a few of the references seen in the technical press:
1. “On average, 30% of traffic on the road during the morning peak is people searching for a parking space”.
2. “Studies have shown that 30% of city traffic congestion is caused by people looking for somewhere to park”.
3. “More than 30% of traffic problems in a city are caused by drivers searching for a parking spot”. [Ms Onesirosan-Martinez referred in her article to a 2011 IBM Global Parking Survey as the source of this figure; and later quoted the same 30% statistic from a 2013 Cisco and Streetline Innovate for Smart Parking project]. However, a quick look at on-line references to these surveys provides no evidence of any rigorous studies on the point in question.
[And, to provide some balance on sources, the IBM survey and figure of 30% was also reported - without comment - on page 11 of Parking Review in December 2011].
4. “Further studies indicated that 30-45% of the traffic volume in urban areas is generated by drivers looking for parking spaces”.
5. “It’s now a well known statistic that drivers spend 40% of their time looking for a parking space – generating a good deal of fumes”.
6. “Experts estimate up to 30% of traffic in congested urban areas where street parking is in high demand results from drivers looking for parking”. Whilst this reference does no more than repeat the figure of 30 %, at least it has the merit of referring to congested areas and to the relevance of the level of demand for parking spaces.
In June 2015, following the publication of the first of the references quoted above, Transportation Professional published (on page 10) a letter from me entitled “Are so many motorists really searching for a parking space?”
And in September 2015, Transportation Professional published (on pages 10/11) a further letter from me entitled “Fewer people hunt for a parking space than has been claimed”. This followed receipt of information on this subject from Joseph Hough and Phil Goodwin (see below).
Regretfully, it would appear from the various articles referred to above, that this correspondence has not been widely seen or taken note of. Hence the reason for this article.
So, what have I found out?
Firstly, the best summary of this subject that I can find – sent to me by Phil Goodwin - is found in a 2013 article by Paul Barter in Reinventing Parking–
Reference is made in this to the work of Donald Shoup, author of “The High Cost of Free Parking”. Shoup referred to a small number of studies (see below), all but one a US source, carried out between 1927 (sic) and 2001. These showed a wide variety of results, from 8% of traffic looking for a parking space to 74%.– this latter being an outlier from the only non-US source - a 1977 study from Freiburg in Germany.
To illustrate the point further, a summary of the results of these earlier studies follows –
1. Two studies in 1927 referred to locations in Downtown, Detroit where it was found that between 2pm and 6pm the amount of traffic cruising for parking was 22% in one study and 34% in the other.
2. A 1960 study of CBD (central business district) traffic at three different time periods in New Haven and Waterbury, Connecticut showed that at least 17% of the traffic was parking search traffic.
3. The 1977 study in Freiburg, referred to above, was based on 800 tracked cars. [NB. No further location or time period data are provided.]
4. A 1985 survey of the Harvard Square business district of Cambridge, Massachusetts between 10am and 3.30pm estimated that 30% of cars were cruising for parking.
5. In 1993 survey between 8am and 10am and 1am and 2pm in Mid-Town (West Side) New York found 8% of traffic was searching for parking.
If one averages the above percentages (which statistically is very bad practice), one comes to a figure of 30.8%. So, it is likely that this is where the oft quoted figure of 30% comes from.
The Reinventing Parking article also referred to two later studies in New York. One of these, in 2006, referred to Prince Street, Soho which found that on Tuesday, Friday and Saturday afternoons/evenings an average of 28% of intercepted motorists at traffic lights were looking for parking in the area. On Saturdays, the share was 41%. [Authors note: The assumption therefore is that on Tuesdays and Fridays, the figures were somewhat less than 28%].
In the other New York study, this one in Park Slope, Brooklyn in 2007, it was found that over four time periods (peak and non-peak) overall 45% of traffic was found to be cruising for parking.
Most studies appeared to be from areas with (in today’s terms) poor parking management policies, some with very low on-street parking charges compared with nearby off-street parking. In other words, the studies were in areas where researchers could expect to find high estimates of drivers searching for on-street parking. They would not be carrying out the studies if they did not perceive a problem to exist.
A second reference sent to me – this one from Joseph Hough – was an article by Donald Shoup in the New York Times in 2007.
This again discusses the effect on congestion of traffic searching for a parking space. In particular, it discusses the effect of cheap on-street parking on the level of congestion. [Authors Note: I would expect a similar effect – ie more searching - would result where the amount of parking space available is less than the demand – but I have not seen specific reference in the literature to this cause and effect].
Readers will probably be aware that Shoup has long-argued, for various reasons, for proper control of free or cheap parking spaces. [See, for example, the report on his views by Peter Guest on pages 26/27 of Parking Review, October 2011].
Shoup refers in the New York Times article to US cities adopting policies of setting on-street parking prices so that occupancy is restricted to 85% of spaces. In ideal circumstance, charges would vary by time of day to achieve these aims.
This would mean that one in every six or seven spaces would be vacant, thus reducing the need to drive around searching for a space. Incidentally, this was a target recommended in the UK at least as far back as 1985.
I started by querying whether there was any validity in saying that 30% of traffic could be searching for a parking space. I believe I have demonstrated that any such estimate is meaningless unless related to specific locations and circumstances. And that locations and times with high estimates of searching traffic are very much the exception rather than the rule.
The available evidence reviewed in this article relates mainly to US studies, to studies in known congested areas and to `old’ studies. How can we include studies from the 1920’s in any serious study of today’s conditions? We could also query the validity of using studies from the 1970’s and 1980’s.
As stated, the “30%” statistic was primarily based on US studies. Whilst parking problems are universal, parking policies and practices certainly are not. In the UK – and all but one of the published quotations I have referred to in this article are from UK journals – we have a long history of parking control and reasonably good enforcement (to ensure a turnover of parking spaces). So, this is another reason not to use such `imported’ statistics.
More serious is that the “30%” statistic appears to be brought out without, apparently, a great deal of thought, to support policies relating to new methods of parking management/control, and without any justification of it’s relevance to the proposal or area involved. [I fully accept that new technology may well result in “searching for a parking space” becoming less of a problem over time. But I think we are 20 years off this utopian situation. Maybe this will be part of the future, when the car drives us rather than the other way around!].
Simply quoting and requoting the 30% (or any similar) statistic has no place in our professional literature, without the necessary careful thought and justification. Hopefully, we will see no more unqualified references to the 30% figure.
Finally, I would like to add my own piece of `research’. Without any surveys, I can confidently say that in my own road – an urban residential road but not a congested one – 99% of the time 0% of the traffic is searching for a parking space. How can I say this? 40 years of observation.
 Parking implications of city network considered, Dr Peter Reynolds, page 12, Transportation Professional, April 2015
 Intelligent parking trials prove successful, Jules Hollows, page VIII in the Innovation Special, Transportation Professional, November/December 2015
 All together now, Christina Onesirosan-Martinez, page 33, Parking News, November 2015
 Engagement: it’s time to commit, Dr Guo Chao Alex Peng, page 26, Parking News, March 2016
 Parking saves the planet, (unattributed), page 41, Parking News, July 2016
 From a ParkJockey/PayByPhone `advertorial’ on page 6 of Parking Trend international, Volume 30, no.1-2016
 Highways and Traffic Management in London, page 5.13