The opponents of a bypass prefer thru-town traffic with its noise, congestion, barrier effects on pedestrians and bicyclists, safety hazards and air pollution?
The key to a successful bypass is protection of access from developments sprouting along it and between it and the town. Where this cannot be secured, the bypass opponents are justifiably fearing land developoment impacts.
On a bypass protected from feeding from adjacent developments, the town businesses do not necessarily loose. First, those motorists interested in the town will drive in to see it and perhaps to buy refreshments. If not, they can still be captured at an aesthetically designed services complex (restaurant, motel, gas stations, point of tourist interest) located on the bypass and run by the town businesses.
Successful bypasses exist in many countries and much could be learned from there. Simply opposing a bypass will lead to other social and environmental problems as the traffic grows. Many negative examples of this approach exist in British Columbia. One is given below (from a 1996 draft of my report titled "Monetization of Environmental Impacts of Roads") .

Salmon Arm Bypass

Salmon Arm municipality occupies an area of 188 km2 in the
interior of the province, with many unique environmental
conditions existing within a mix of urban and rural
characteristics. Quality of life is important to the people of
Salmon Arm. It encompasses a combination of small town
atmosphere with many quality urban-style, sophisticated
services and features. The theme for Salmon Arm's 10-year
development strategy envisions "livability through respect for
nature balanced with the need for jobs". The people of Salmon
Arm want an integrated approach to transportation planning,
including pedestrian and bicycle links through the Trans
Canada Highway that bisects the community (District of Salmon
Arm 1992).

A railway is also a major transportation influence in Salmon
Arm. It acts as a major barrier to access to Shuswap Lake; and
any lakeshore development requires maximum safety precautions
at crossings. The railway causes noise, safety (including
transportation of hazardous goods), and visual impacts on
residents and visitors.

In the long term, the community wished for a highway bypass
route to separate local and inter-regional traffic from the
Trans Canada Highway, but the Ministry's regional unit failed
to respond to the community needs. Although there are good
historical reasons that the national highway and railway sever
the community, bridge technology is now available to cross a
1700 m wide lake to bypass the community. One bridge can
accommodated both rail and road.

Piotr Bein, P. Eng.
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> Subject:        Town Bypass = Suicide?
> From:   anonymous <[log in to unmask]>
> Date:   Sat, 28 Feb 1998 15:47:33 -0500
>
> We are looking for information about the damage to small towns created
> by the construction of bypass roads, such as decline of in-town
> businesses and increased sprawl and ugly development in the surrounding
> area.  Does anyone know of any examples of towns that have suffered in
> this way?  We are going to be making a presentation to the county
> commissioners and town council within the next few days about how the
> proposed bypass around our town might not be such a good idea.