A car stationary but left running (idling) will cause pollution in that specific location; start stop technology could avoid the pollution responsible for idling. Here I will concentrate not on the global impact of the pollution but at the localised affected area. The differing scenarios are important to understand how much of an effect start-stop will have on air quality.
Taxi ranks, junctions, city centres, high streets, spaces outside schools, hospitals and other areas where cars often stop for longer than a few seconds; the important part is that these are usually places with a large number of people. The longer you idle and the more cars doing it, the more pollution is created.
I present a few scenarios (a link at the bottom for my calculations)
A long journey with minimal stopping
Here the savings on pollution would be negligible because the car would not be stopped idling for much time at all! Although of course the locations where the car does stop would be subject to fewer pollutants.
Major junction at 11:30am on a weekday (figures taken from Preston Circus Brighton)
During a full cycle of all traffic lights going from red to green and back for all junctions, 69 cars passed during 112 seconds. The cars used an estimated 0.451L of petrol. During this time 32 cars waited for 30 seconds using 0.2L of petrol. Idling in this scenario is responsible for 44% of the fuel used and therefore I assume idling is responsible for 44% of the pollution.
Method: Cars waiting count - I counted all the cars stopped just before the lights turned red and timed the total for the red light for each traffic light. I then divided that number by two assuming the cars waited on average half the time.
32 cars waiting an average of 30s (adjusted 10s for startup)
69 cars passed this junction over 112s
Length of junction (with a bit either side) = 65m
Total fuel used whilst idling
0.00021L/s x 32cars x 30s = 0.2L
Total fuel used driving along junction
1/(10000m/L x (1m/65m)) x 69 cars = 0.451L
A major city centre junction like this is a place where pedestrians will wait at crossings, drivers will sit in their cars, cyclists will stop to catch their breath and where pedestrians sometimes have no alternative route.
44% here is a huge and I think a very significant number.
Single car waiting in a single spot
In a car you would use around 0.75L of petrol per hour idling and you would change that single location from being pollution free to a place equal to a single lane road with regular traffic (1 car every 12 seconds)
A 1.5km stretch of road with 4 major junctions with traffic lights
A single 1.5km stretch of road could have four major junctions in a busy city. A car stops twice for a combined total of 2 minutes. The car will use around 0.025L of petrol idling. Driving along this stretch of road would use around 0.15L of petrol, assuming the amount of fuel is equal to the pollution then start stop could reduce pollution by 6%. This is a fairly small amount compared to the total but it is still significant difference in the air quality along a large stretch of road. Streets like these in city centres will have many more people than in other areas waiting at the lights, walking along pavements, people working in shops, people living in flats above the roads, cyclists and drivers will be in their cars taking in the pollution. These areas are some of the most dangerous generally for people because of the number of people so 6% could make a significant difference.
A taxi rank
A taxi rank with 10 cars idling in a space of around 35m. That 35m spot could have the air quality equal to a reasonably busy dual carriage way (a car every 1.68 seconds). Some taxi ranks are covered (such as those near train stations) which could make those spaces much much worse as the pollution would not be dispersed easily. There will be people waiting for taxis next to the taxi rank, people waiting nearby as taxi ranks are usually in busy locations and the taxi drivers themselves will be sitting directly in these toxic fumes so there will be many people exposed to this pollution. With start-stop, this pollution would be completely eradicated.
The cars could be spread out more so you could say it would be a half of the above, so a busy single lane A road maybe. The affects of pollution are greater on children because their lungs are smaller, more sensitive and growing. Children and babies in prams are also more affected because they are shorter and closer to the pollution.
Amount of fuel used per hour idling Cost of engine restart
Fuel used idling = 0.165 gallons/h = 0.75L/h = 0.00021L/s
I assume 23.5mg = 10km/L = 10,000m/L
I assume each restart is equal to 10 seconds running (more accurately 3s)