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Diverting traffic from main artery to neighborhood road


The city that I live in is planning to build a road through a four-subdivision area, connecting a county highway (4 lane, 45 mph) and a local city street (2 lane, 30 mph, but 20 mph at certain hours during school days). The objective is to reduce traffic on the county highway by providing traffic from the subdivisions with an alternate entrance/exit. At times, both roads are congested.

Many residents are trying to stop this project but the city government is persisting.

I am looking for two kinds of information to bolster our case against this project.

The first is some kind of authoritative guideline (perhaps something called a rule of thumb, bible of road planning, ten commandments of city development, manual for road planning, government standards for new road planning, etc) that says unequivocally something like this:
“Never, under any circumstances, construct a new road that will tend to divert traffic from a major artery onto local neighborhood streets.”

Any ideas?

The second thing I am looking for is a quantitative relationship to back up or refine my intuitive idea that if a given amount of traffic on a 4-lane high-speed highway is diverted to a 2-lane low-speed road, it does more harm. In other words, it is more likely to cause traffic jams, accidents, etc. For example, is there a rule of thumb that says if the diversion of traffic decreases the traffic by X percent on the main artery, it will increase traffic by say Y percent on the local road, where Y= X times some factor which is a function of the two speed limits, number of lanes, etc?

The reason I am looking for this info is that some people have the attitude that a road is a road, it is ok to make one worse if you are making the other better. It seems intuitive to me that a road is not just a road; the diversion is likely to make the county road just a tiny bit better, while making the local road much worse.

Thanks for any ideas.



Dear Leader
Staff member
I'd have to see a map of the area to get a clearer picture, but ... right now, there's two prevailing, but diametrically opposed practices about distributing traffic in the planning world.

The traditional practice in North America -- really, most prevalent from the 1930s through the late 1990s -- favored a strict hierarchy of street functions. Ttransportation planners and engineers usually classified streets as major and minor arterials, collectors, and local streets. Local streets fed into collectors, which fed into arterials. Some call this a "dendritic" pattern. This ensured traffic on local streets would always be light, but it limited routes between one destination and another, meaning streets downstream would always be busier. Also, it indirectly locked in land use patterns, since it would be impractical to build a more intensive commercial or multi-family residential development on an out-of-the-way local street.

The second school of thought is that more connected street patterns, whether they're in the form of a grid, gently curving blocks, or Boston-like cowpaths, offer a more holistic way to deal with traffic. With higher connectivity, there's more than one way to get from Point A to Point B, which means traffic doesn't need to be funneled to one collector. Higher interconnectivity also makes an area more adaptive to future growth without rearranging streets-- consider that the downtownss and neighborhood centers of most American cities started off as primarily residential villages. Although traffic is distributed along more streets, it'll also move a bit more slowly. -- provided traffic engineers don't turn them into one-way racetracks.

This image is a bit of a cliche, but it does a good job at comparing the two approaches:

I'm sure there's an approach to determining the long-term financial costs vs benefits of adding a reliever road -- time saved, accidents reduced, increase or decrease of real estate costs, etc. vs construction and maintenance costs, loss of taxable real estate. However, you can't always put a monetary value on the "public good" -- the public health, safety, and general welfare justification you'll see in so many laws and resolutions. If we always took the cheapest approach, we'd all be living on dirt roads. I'd be looking at a more holistic approach -- not just better ways of moving cars, but how to create a great place in the process of whatever solution they come up with. A reliever road and traffic counts alone shouldn't be an indicator of livability. Nonetheless, is the congestion on the county road so bad that it justifies spending millions of dollars on a solution that may or may not work?

Anyhow, back to your question. Is the county highway a commercial corridor? If it is, the reliever would function as a bypass. Communities love the idea of bypasses, for the traffic relief they bring, but they don't consider their costs. The bypasses around small towns kept 18-wheelers from clogging up downtowns, but they also kept through traffic away, too, hurting businesses. Eventually, businesses would relocate to the bypass, where the traffic was. The outcome: a dead downtown, a long suburban strip far beyond walking distance of where most town residents live, and increased traffic.

Also, there's the phenomenon of induced demand. In plain English, "build it, and they will come". If a relief route speeds up traffic, it'll attract more traffic, and you'll eventually be back where you began.


Thank you for your insightful reply, which has helped me see the situation from a different perspective.

In response to your questions, let me try to fill in some of the specifics.

Things here are laid out in the old boxy style. There is a group of four essentially rectangular subdivisions arranged inside a larger rectangle. Each subdivision is a gated community, with a single access to an existing road that goes eastward from the county road to the center point of the larger rectangle. The new road will go from that center point to the east boundary of the subdivision, then turn north and intersect with the local road.

The city owns the property the new road will be built on because it was given to them years ago; and the road will be short (on the order of a half a mile). The money is available, so the city is not being forced to provide much justification to taxpayers.

I am never sure of my terminology on these issues, but I would characterize the county road this way: There are several housing developments nearby, several new developments under construction, shopping centers (a few small, one huge), small businesses, a post office, schools, etc. The traffic engineers call it a divided four-lane arterial.

The local street is residential but has itself become a cut through for drivers looking for a short route to nearby businesses. The local road already has a traffic congestion problem that occasionally blocks access from side streets. The traffic engineers call it a two-lane arterial roadway, which I don’t know what to make of.

Perhaps an important thing to understand is that all of this was farmland only a few decades ago, and there is something like panic going on here (on both side of the issue) because of the uncontrolled growth, and fear of waves of traffic in the future. I think we all have the attitude that within a few years, regardless of what current traffic studies or counts may say, we are going to be drowning in traffic.

Residents have requested a new traffic study but the city does not have a budget for actual traffic counts, so they will be using traffic modeling software and existing data to model the impact. Residents seem skeptical.

Residents have requested a cost benefit analysis, as you suggested, but the city has resisted.

Also, the situation is more or less binary - we are not laying out a city from scratch. All the land has been gobbled up, the choices are to build the road or not, though of course there may be some possibilities for traffic calming measures.

Speaking of which, I have found that a lot of cities have "Traffic Calming Plans". An interesting theme that seems to pop up often in these plans is a precaution against new measures which may (unintentionally) divert traffic from main arterials to local neighborhood streets. That is what led me to search for some general rule on the subject. (ANY THOUGHTS ON THIS?)

At any rate, our situation is rather similar to having a cul-de-sac in a housing area, and constructing a cut through street from the cul-de-sac to the outside world. Interestingly, I recently found a study* which shows that when a new road is built through an existing neighborhood like that, the cut-through traffic from adjacent major roads can be quite heavy. Also, the cut-through traffic tends to travel at higher speeds, leading to more accidents; and the accidents tend to involve more fatalities to motorists, cyclists, and pedestrians. (ANY THOUGHTS ON THIS?)

If you have any comments on any of that, I would be interested.

Thanks again.


Comprehensive Engineering Approach to Achieving Safe Neighborhoods
James A. Bonneson, Angelia H. Parham and Karl Zimmerman
Texas Transportation Institute
Texas A&M University System