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Stop Sprawl
Traffic Calming Cleans

New Emissions Assay: Freeway Growth Pollutes; Traffic Calming Cleans

Dr. John Holtzclaw

I'm following up on my 5 May 99 memo that reported preliminary results of new motor vehicle emission tests at UC Riverside and Georgia Tech, which followed tunnel measurements showing emissions 60% higher than existing emission models predict. Final results of the studies indicate that:
1. highway expansions to relieve traffic congestion actually increase motor vehicle emissions, whether they free up traffic or not (this conclusion does not apply to transit-only lanes, which have the potential to reduce auto traffic), and
2. engineering treatments and traffic enforcement to slow traffic in neighborhoods - traffic calming, calms emissions.

These conclusions have not changed, but first some air pollution background. The UC Riverside and Georgia Tech studies have been used to develop new emissions models (emissions factors): EPA's MOBILE6 and California Air Resources Boardıs EMFAC2000. CARB reports that Californiaıs actual motor vehicle emissions of hydrocarbons (HC) are higher than previous estimates by 78%, nitrogen oxides (NOx) by 68% and carbon monoxide (CO) by 93% (CARB, "Public Meeting to Consider Approval of Revisions to the Stateıs On-Road Emissions Inventory Estimation Model, EMFAC2000," November 1999). Motor vehicle emissions are crucial: in the Bay Area, for instance, motor vehicles emitted about half the criteria pollutants (HC, NOx, CO, particulates) with the old estimate, and probably about 2/3 of HC and NOx with these revisions.

All right, I hear some saying "We're not CA". That may be; but you will be. NY, Conn. and Mass. have already adopted CA emission standards, and EPA is tightening up federal standards. Also, older CA cars emit more than newer cars, as elsewhere, due to being built to lower pollution standards and due to the insults of age and wear. EPAıs MOBILE6, for the whole country, reflects the same emissions reevaluations.

CARB's report estimates that in 2000 the LA motor vehicle HC emissions will be from:

Passenger cars 55%
Light trucks (SUVs, pickups, vans) 22%
Medium trucks 17%
Heavy - gas 3%
Heavy - diesel 2%
Bus 0.1%
Motorcycle 1.3%

CARB's estimate of motor vehicle HC emissions sources in 2000:

Tailpipe (cold starts, running) 53%
Running losses (fuel & exhaust system leakage) 22%
Hot soak (cooling off after trip) 18%
Diurnal (baking in sun, cooling at night) 5%
Resting (drip, drip, drip) 2%

This shows that some 75% of HC are emitted during driving, 18% as the engine cools down afterwards, and 7% whether it is driven or not (as long as it has fuel). So these new tailpipe emissions factors affect 53% of total motor vehicle emissions.

Emissions increase with speed. Not only are total tailpipe emissions larger than previously assumed, but the shape of the curve of emissions at different average speeds is changed. Off-the-road cars and light trucks were tested for emissions at different speeds and rates of acceleration. They were also tested with speed profiles developed by on-road freeway and arterial driving during various levels of congestion. These profiles are more representative of actual driving, including more aggressive driving and higher freeway speeds (up to 75 mph at free-flow conditions, compared to 57 mph), than the profiles used in earlier emissions models. The vehicles were clustered by vehicle age, type and catalyst, and tailpipe emissions measured for average freeway and arterial road speeds.

The new tests show higher HC emissions than the old model. For 22 of the 26 vehicle clusters, the emissions increase dramatically at speeds above 60 mph, reflecting large increases of emissions during accelerations when the engine is under high loading. The four clusters showing reduced HC emissions at higher speeds are 20+ year old cars with no catalyst, pre-1979 light trucks, 1979-1983 light trucks, and diesel trucks. The emissions impacts of these aging older vehicles will be reduced as they are removed from service. Emissions from diesel trucks are being tightened, and the change of new diesel engines emissions with speed is not now known.

Actually, this increase in emissions at higher speeds has been known for years, but not adequately incorporated into emissions models because of the low maximum speed assumed. Even now, the higher emissions at higher speeds have not yet been fully incorporated into MOBILE 6 or EMFAC2000.

Chris Brittle of the Metropolitan Transportation Commission confirmed these conclusions about high-speed emissions, even with the older model. At a 4 May 1999 meeting of the Bay Area Air Quality Management Districtıs Advisory Council, Mr. Brittle said that MTCıs calculations showed that holding maximum highway speeds to 60 mph would reduce Bay Area mobile source emissions by 25 tons/day, 14% of the predicted year 2000 on-road emissions. Further, he reported that holding the maximum highway speed to 55 mph would reduce emissions by 35 tons/day, 20% of the on-road emissions.

The new tests also show mixed (some increasing and some decreasing) emissions per mile at freeway speeds below 20 mph, reflecting very congested roadways with occasional hard accelerations and braking. There were no tests of emissions during the mild conditions encountered on "calmed" slow-speed neighborhood streets because emissions are so low that they are unimportant to regulators (Matthew Barth, et al. "Comprehensive Modal Emissions Model (CMEM), version 2.0 Users Guide". University of California, Riverside. 3 January 2000.).

Besides HC, the other important ozone precurser are nitrogen oxides (NOx). NOx reacts with HC in sunlight to form ozone or smog. The NOx emissions follow the same general pattern as HC. Of the 25 clusters of vehicles, all except diesel trucks emit more NOx per mile as speeds increase, especially above 60 mph. So the above conclusions about HC emissions apply even more strongly to NOx.


Many freeway widenings or extensions are sold as measures to relieve traffic congestion, thereby reducing emissions, at least as projected by plannersı old emissions models. Sometimes the builders promise that the new lanes will be HOV (high occupancy vehicles, probably for a few hours of the day, the rest as mixed-flow; with the very real threat of conversion to full-time mixed-flow). What light do these new assessments shine on that argument?

Widened freeway. Let's start with a garden-variety freeway -- 2 lanes in each direction, and congested during rush-hour in at least one direction. So our helpful state DoT adds a lane in either direction. The immediate changes are:
- traffic speeds up a bit in all lanes as traffic shifts to the new lane, with less traffic shifting if the new lane is HOV.
- additional, induced, traffic eats up much of this new capacity as motorists perceive congestion relief and (1) opt to take a trip they otherwise wouldn't have, (2) take a longer trip, (3) change to this road from another, (4) change mode (drive rather than carpool, transit, bike or walk), or (5) move farther from work (adding to sprawl?), or choose a job farther from home.

Noland ("Relationships Between Highway Capacity and Induced Vehicle Travel," Transportation Research A, forthcoming) found that 20 to 50% of such new road capacity is immediately filled. Hansen and Huang ("Road Supply and Traffic in California Urban Areas," Transportation Research A, 31:205-218) found that 90% of the new road capacity was used up with new traffic within 4 years. Subsequent studies have confirmed these impacts, see Sierra Club, Victoria Transport Policy Institute , and Surface Transportation Policy Project

So what happens when the new freeway lane is opened? Traffic quickly rises to fill 20 to 50% of the new lane capacity, giving a 10 to 25% increase in total traffic. If the new lane is mixed-flow, the increase in total traffic will reduce traffic per lane 25 to 30%, increasing speed AND emissions per vehicle slightly (Barth, Scora and Younglove, "Estimating Emissions and Fuel Consumption for Different Levels of Freeway Congestion," TRB Meeting, Jan. 1999). Which, when added to the emissions from the additional traffic, increases total emissions by more than the 10 to 25%.

HOV construction. Even if the new lane is designated HOV and enforced, so it attracts fewer vehicles and has free-flowing traffic, emissions from this high-speed traffic would skyrocket (Barth, Scora and Younglove). Again, total emissions would increase by more than 10 to 25%. Even without the induced traffic, the emissions tests show a clear pattern of level, or slightly rising, emissions per vehicle as speed increases until the average speed reaches 50 to 60 mph, above which emissions skyrocket due to accelerations, even short accelerations, under high engine loads.

Over four years of so, traffic rises in each of these lanes almost to where it was before the road was widened, with a net increase in emissions of almost 50% from before the road expansion.

New freeway. What happens when a new freeway opens? Anticipation of it probably would have encouraged many moving decisions even before it opens, driving up sprawl and longer trips. On opening, expect the 20 to 50% additional traffic. If some lanes are free-flowing, emissions from these cars will skyrocket. Total emissions would be up more than the 20 to 50% new traffic, and over the next four years traffic and emissions would be up more than 90% from before opening.

What about the other ozone precurser, NOx? The tests show that it follows a very similar pattern of emissions with speed as HC. So the above conclusions also hold for NOx.

Final results indicate that highway expansions to relieve traffic congestion increase motor vehicle emissions, whether they free up traffic or not (this conclusion does not apply to transit-only lanes, which have the potential to reduce auto traffic).


Traffic calming consists of engineering treatments and traffic enforcement to slow traffic in neighborhoods. These include narrowing roadways and lanes, center landscaping, shorter blocks, widening sidewalks into the roadway at cross-walks and intersections, tighter turns at intersections, occasional necking-down of the roadway, and other improvements to reduce maximum speeds and lower accelerations and decelerations (Burden, "Street Design Guidelines for Healthy Neighborhoods," Center for Livable Communities, Jan 1999; ).

Conventional wisdom predicts that this slow, stop and go traffic will vastly drive up emissions, choking the neighbors. But conventional wisdom is ignorant of traffic calming, confusing it with fast starts and stops on conventional collector and arterial streets, and on congested freeways where motorists are jockeying for position. And, as it turns out, conventional wisdom is ignorant of emissions in those situations too.

Conventional wisdomıs high estimate of emissions from stopping and starting is a bum rap since the old emissions factors "at lower speeds generally overpredict emissions" (Barth, Scora and Younglove, p12). Where does this misconception come from? Perhaps because cold starts, which normally occur at a very low speed (parked), have high emissions. Or perhaps because old diesel trucks and buses emit a very visible particulate cloud when accelerating. Cold starts occur at the beginning of every trip, fast or slow, whenever the vehicle has been allowed to cool down. Diesels are, fortunately, a small fraction of the vehicle fleet, although a noisy stinky one.

Professor Randall Guensler of Georgia Tech suggests that this misconception comes " from our historic use of the nonlinear relationship in MOBILE and EMFAC first. Every government public information publication contains this curve. Plus, since we tend to think of emissions in grams/mile, most people can conceive that if traffic is standing still the emissions per mile skyrocket. With respect to cold starts and diesel, I would further emphasize that the perception arises from our sitting in congested traffic.
1) we don't like to do this,
2) we are actually impacted by higher concentrations of emissions and the public generally associates this with higher emissions rates rather than reduced mixing and cumulative emissions, and
3) we can see the visible emissions associated with diesel acceleration, cold starts, and high emitters."

However, the new assays show that accelerations at low speed increase emissions of either HC or NOx much less than at high speed because the engine is under much lower loading. Still, smoothing flow with traffic calming will reduce emissions even farther. Obviously, these conclusions are not meant to cover cars sitting idling all day without moving; they will have infinite gms/mi emissions (division by 0).

Traffic calming has been shown in practice to reduce emissions. Traffic calming in German residential areas reduced idle times by 15%, gear changing by 12%,braking by 14% andfuel use by 12% (Peter Newman and Jeffrey Kenworthy. Sustainability and Cities: Overcoming Automobile Dependence. Island press: Washington DC, 1999. p150.). And they report that other German tests showed a 10% reduction of HC and 32% reduction of NOx emissions when aggressive drivers average speed is reduced from 31 to 19 mph, and 22% and 48%, respectively, when calm drivers average speeds are reduced.

These new emission studies indicate that engineering treatments and traffic enforcement to slow traffic in neighborhoods -- traffic calming, calms emissions. Dust off those traffic calming schemes, bicycle lanes, stop signs and transit-only lanes. They reduce emissions and traffic hazards for pedestrians, bicyclists, transit riders and even motorists.

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