|Photo by Clive Perrin|
This post offers some answers and gives you tools to find out more.
Basically, to calculate the percent of time that cars are parked we need to estimate the number of hours that the average car is in motion. After that the time spent parking is simple.
Give it a try for a place you care about! And when you are done, please share your results (and your assumptions) in the comments!
Here are three different approaches to the calculation. There are probably others, which you may be forced to try depending on the data available.
Option #1: based on the number of cars, the number of car trips and the average time duration of car trips:
A UK report on parking put out last year by the RAC Foundation (and well worth a read by the way!) uses this method based on data from the UK National Travel Survey (NTS) (see p.23):
"... there are about 25 billion car trips per year, and with some 27 million cars, this suggests an average of just under 18 trips per car every week. Since the duration of the average car trip is about 20 minutes, the typical car is only on the move for 6 hours in the week: for the remaining 162 hours it is stationary – parked."
Since there are 168 hours in a week, the typical UK car is parked 96.5% of the time - even higher than Shoup's US estimate!
With this method, be careful to use car trips and not just trips by car. In other words, you want trips by car as driver so that you don't count trips as a car passenger. If you only have a total for both drivers and passengers then you will need to find an estimate of overall average car occupancy and divide by that to get the vehicle trips (driver trips) number.
Option #2: Based on time drivers spend driving (from transportation surveys) and assuming one car per driver.
If your local travel survey spits out a number for time spent driving then you can use that directly for a rough estimate.
This is how Shoup gets his 95% number. See Appendix B (p.624) of his epic tome "The High Cost of Free Parking". He cites the 1995 Nationwide Personal Transportaton Survey (NPTS) of the US Department of Transportation as finding that the average time drivers spent driving was 73 minutes (1.2 hours). Assuming one car per driver (which is roughly OK probably for the US context), this gives 5% as the time each car is in motion.
Option #3: Using car kilometres per car and overall average speeds
Transportation studies for metropolitan areas often provide data on the average yearly distance driven per car and the overall (24 hour, 7 days) traffic speed. If so, you can use this easy method (although note that those 24/7 speed numbers may not be the most solid of transport statistics I suspect). If you don't have car km per car you may be able to get it by dividing total car km by the number of cars, being careful that the two numbers are for the same study area.
The average time each car is in motion is the car km per year divided by the average speed.
I took a look at the 1995 UITP Millennium Cities Database numbers using this method. The average percentage of time that cars were parked for the 84 cities in that study was 95.8%. They were typically in motion for 1.02 hours per day.
For example, for Singapore in 1995, we have a 24/7 speed of 35.2 km/hr and 18,486 car km per car. This gives 525 annual hours of operation per car and, since there are 8,760 hours in a year, we get 94% as the percent of time a 1995 Singapore car was spent parked.
The lowest figures (88.4% and 89.4%) were two Chinese cities with suspicious looking speed numbers (nice round 18 km/h for both, which looks like a rough estimate and on the slow side). The next lowest figure was 92.3% in Seoul with 16,013 car km per car and 23.8 km/h in 1995.
First, I have confirmed that Shoup's estimate of 95% does seem widely applicable. Across the world cars seem to be parked at least 92% of the time and typically about 96% of the time, according to the 1995 data mentioned above. I doubt more up to date or accurate data sets would change this number much.
But why should we care?
One reason to talk about this is to highlight the importance of parking. It is what cars do the vast majority of the time.
It highlights a crucial inefficiency of mass private car ownership. It points towards huge parking space savings (an enormous land bank) that shifts away from mass car ownership might open up, if only we could massively improve the alternatives including making car-sharing and other 'metered access to shared cars' (MASC) more of a mass market phenomenon.
The numbers on parking time also tend to surprise people, which makes them worth mentioning.
But of course, once you think about it, we shouldn't really be surprised. It WOULD be surprising if privately owned household cars were typically in motion for any more than 1 hour 40 minutes which means parking would be less than 93% of the time.
What if you exclude sleeping time, say the 8 hours from 10.30pm to 6.30am? Even then in-motion time is only 1.02/16 hrs = 6.4%. So cars are typically parked for 93.6% of even WAKING hours!
So once again, give it a try with numbers on the places you care about.