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Road Projects

4 lanes to 3 lanes FAQ

What is a 4-lane to 3-lane conversion?

This means restriping a road with four lanes- two in each direction- as a road with three lanes- one lane in each direction with a center turn lane. Engineers and planners alike have found that in high turn environments (like Broadway Street here in Missoula) three-lane roads can carry as many motor vehicles as a four-lane road- with greater safety and efficiency for all modes of transportation.


How can a road with fewer lanes carry the same amount of traffic?
When a car stops in a moving traffic lane to turn left- especially common in downtown environments- this leads to: congestion, blind spots, unsafe lane changes and variability in vehicle speeds.  These results often results in crashes. In a three-lane system there is always one lane for driving, and one lane for turning, no mixing up the two.  This makes driving safer and more reliable, with fewer crashes and frustrations. For these reasons, a 3-lane road can handle the same amount of traffic, or more, as a 4-lane road.

Another part of the congestion equation is the intersection. The intersection often determines the levels of congestion on a road.  When converting a 4-lane road to a 3-lane, the number of total lanes at the intersection usually continues to be the same.  There is much potential for further enhancing traffic flow and safety by using single lane roundabouts instead of traffic signals at the intersections.  Single lane roundabouts can not be used effectively with 4-lane roads, in our opinion (the two entering lanes would have to merge to one lane).  This puts even more importance on converting 4-lane urban roads to 3-lane urban roads- we can then utilize single lane roundabouts.  A 3-lane roadway with single lane roundabouts typically operates much better than a 4-lane road with signals.

How does a 4 to 3 conversion make driving safer?
4 to 3 conversions provide a center turn lane so that left turns are simpler. A driver crosses only one lane of traffic at a time (resulting in fewer blind spots). With an undivided 4-lane road, a driver must find a gap in two or three lanes of traffic at once to make a left turn. 

How does a 4 to 3 conversion make walking safer?
First, you only have to cross three lanes of traffic, not four. Second, there are fewer blind spots as you only have one lane in each direction, thus there is less sight blockage by cars. Third, top vehicle speeds in a three lane system are lower (this does not mean that it will take longer to get through downtown on Broadway, it means that there will be less speeding up and slowing down, and a more consistent pace).  The most important factor that increases pedestrian safety on a 3-lane compared to a 4-lane: on a 4-lane roadway, when one car stops to let a pedestrian cross, often it’s the other lanes that do not stop which leads to severe crashes, while with a 3-lane, when one car stops there is no threat of another motor vehicle coming on the inside lane.

Another way a 4 to 3 conversion can increase pedestrian safety is with the addition of bike lanes.  Most 4-lane urban arterials do not have bike lanes- the space gained by converting to a 3-lane allows bike lanes (usually six feet wide) to be added to the road.  With safer on-street cycling facilities, more cyclists will use the road instead of sidewalks, thus leaving the sidewalks to pedestrians.

 How does a 4 to 3 conversion make biking safer?
On a typical 4-lane roadway without bike lanes, like what used to exist on several sections of Broadway, a cyclist is at risk of being 'doored' by someone getting out of a parked car, or being 'mirrored' by a motor vehicle passing too closely (within a foot or two).

By converting to a 3-lane roadway, bike lanes can be added.  A typical 4-lane urban arterial with parking might be 64’ wide (the four travel lanes are usually 12’, plus 8’ parking lane on each side).  By converting to a 3-lane roadway, twelve feet is essentially gained.  This 12’ can be divided into a six foot bike lane on each side.   

Seven feet is a more desirable bike lane width than six feet.  Eleven feet is a more desirable lane width for motor vehicles than twelve feet. With each of the 3 travel lanes giving up a foot (12' to 11'), the bike lanes can go from 6' to 7'.  The extra foot can translate to an extra 6" buffer between the parking lane and bike lane. 

What are the specific traffic lane dimensions?
A   roadway that is currently marked at 8’ parking, four 12’ travel lanes, 8’ parking, can be remarked as: 8’ parking, 6" buffer, 7’ bike lane, three 11’ foot travel lanes (counting the center turn lane), 7’ bike lane, 6" buffer, 8’ parking lane.  If no parking is present, we recommend 6' bike lanes and 11' travel lanes.

Does traffic divert to other streets?
Some traffic may shift routes initially if there is confusion, or if there is an awkward 4 to 3 transition, yet in general there is very little if any shifting of motor vehicles onto adjacent streets. 

What other benefits are there to a 3-lane with bike lanes?
To see why bike lanes can be good for everyone, check out these bike lane benefits.



Why did some people oppose the local Broadway 'Road Diet' project?
Some citizens wanted the $300,000 to go to revamping the intersection of Reserve and Mullan. Reserve and Mullan ended up receiving money from the DOT in 2011 to add a turn lane and go to protected left turns. 

Others citizens thought that there would be more congestion with a 3-lane roadway.  This was a reaction to the concept that fewer lanes for cars can move traffic in a better way.  This has not been the case in the seven years of operations as a 3-lane facility (as of 2012).   Traffic delays have been roughly the same before and after the conversion.  Most current delays are a result of the traffic signals at Toole, Scott and Orange.

Another argument has been that 5-lanes would be better for downtown.   Yet, 5-lane roadways- when compared to 3-lanes- tend to be much less safe to pedestrians and cyclists, take up much more room, cost more to build and maintain, and have higher severe injury crash rates for all modes.

How much did the Broadway project cost?
The City of Missoula cost was $50,000.  The Montana Department of Transportation cost was $250,000.  The project was a restriping, not a redevelopment, so the project was quick with minimal closures, and inexpensive as far as road projects go. Most of the money went into engineering the project, and paying people to paint the roads.

What is the current status of the Broadway project?
Broadway is a mixing of lane configurations: West of Toole St., Broadway is a 5-lane roadway (with a high injury crash rate).  The 3-lane section that was converted from 4-lanes to 3 in the fall of 2005 runs from Toole St. to Orange Ave (almost one mile).  From Orange to Madison Ave., Broadway is an undivided 4-lane arterial, with high rates of car crashes, higer speed, and a history of pedestrians and cyclists being injured.  The road widens to 5-lanes from Madison to Van Buren, and then becomes a 3-lane for 3 blocks, before becoming a 2-lane roadway heading east to E. Missoula.

How can Broadway be further improved?
MIST advocates that all of Broadway become a consistent 3-lane facility, with wide bike lanes (or possibly cycle tracks).  This advocacy is happening through several planning processes: the W. Broadway Vision Plan, the Downtown Master Plan, and the Long Range Transportation Plan, for instance.  We have asked that this roadway conversion be modeled in the 2012 Long Range Transportation Plan update.


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