Bridge numbers don't exactly add up

Posted: Wednesday, October 01, 2003

Curious about the controversy over the proposed addition of a lane to the Douglas Bridge, I decided to do some measuring. The results might interest Empire readers.

I first measured the distance between the railings on the bridge. At the Juneau end it is 36 feet, the width the city and state propose to put three lanes.

Being more interested in usable lane width, I explored the width possibilities for the lane separation lines. I suspect the highway department deals in overall width, but I am interested in what I can actually use. Typical highway lines are four inches wide. The double yellow traffic separation lines are one foot wide. The white lines separating turning traffic from continuing traffic along Egan are eight inches. The white lines at crosswalks are a foot wide and there are two of them, making each side of a crosswalk two feet in width. I looked for some federal highway recommendation on the Internet, but found none. However, I assume the lines will have to be quite wide. I also assume that traffic separation will be assisted by the use of traffic cones. They measure 14 inches across the ends. Therefore, whatever width is painted on as lane separation, be it eight or nine inches, or a foot, the cones provide the practical usage width of over a foot. For the purposes of this discussion, let's say the lane line widths will each be one foot. A foot is near the average of eight or nine inches plus the 14-inch cone width. Therefore, considering the entire width of the bridge at 36 feet, because of lines, the usable width of the bridge is reduced by two feet to 34 feet.

Now we come to the edges of the two outside lanes. How far from the edge of the railings will they be? The Brotherhood Bridge has fog lines that are three feet from the curb to the inside edge of the fog lines. But I assume that that distance accommodates a quasi bike lane, so I don't think that distance would be instructive. As you approach the intersection of Egan Expressway and the Mendenhall Loop Road from the Visitor's Center, the roadway adds a lane. What was once a wide bike lane essentially disappears as the fog line approaches the curb to approximately a foot from the curb. Between the curb and the road asphalt is a one-foot concrete rain gutter. This being the only measurement I can find which I could relate to the Douglas Bridge railings, I have assumed that the there will be a one-foot distance from the Douglas Bridge railings, whether the state paints fog lines or not. Therefore, the usable bridge width is further reduced by two feet (one foot per railing) and the total usable width of the Douglas Bridge is further reduced by two feet to 32 feet.

If the state is planning on a 10-foot-wide middle lane, the two outside lanes will be 11 feet wide, again using practical, driveable lane widths between the lines, not including them. Most lanes are 11 feet 6 inches between the center line and the fog line (at least the ones I measured), so the two proposed outside lanes would be six inches less, and the center lane would be a foot and a half less than what we are used to.

So, I then went about measuring the widths of various vehicles. My truck measures seven feet wide. When I include the rearview mirrors, they add on nearly a foot - six inches per side. School buses are eight feet wide, plus rear-view mirrors. City buses are eight and a half feet wide with mirrors. A large delivery truck measured 8 feet 1 inches plus another one foot for mirrors, making its practical width nine feet wide. Now I realize that most passenger vehicles are below the height of the mirrors on a bus or truck, but mirrors of other buses and trucks are not. The 40-foot-long van I measured was about two inches over eight feet. The Load King earth-mover truck, those large, long ones, are eight and a half feet wide, plus mirrors.

Given the above, suppose two buses are in the outside lanes going in opposite directions. If they are traveling down the middle of their 11-foot lanes, they will be 15 feet apart; the 10-foot middle lane, the two feet for the lines, and a foot and a half from the lane lines to the bus (the eight-foot-wide vehicle in the 11-foot lane leaves three feet distributed one-half on each side of the vehicle). Perhaps the city might park two buses somewhere in town (at a mall?) 15 feet apart and let folks drive between them to give an idea of what that may feel like. Perhaps it could be at a location allowing for bridge speeds.

For me in my pickup at seven feet wide plus a foot for mirrors, I would have three and a half feet between me and either of the buses. That seems like plenty, but this scenario assumes that all drivers are driving exactly in the middle of their lane.

Now 15 feet seems like a lot. But what if the bus/truck drivers are not directly in the middle of their lane and each dips towards the middle of the roadway. Starting off, there would be a foot less leeway than one would find on Glacier Highway. If both of the bus drivers dip in towards the middle of the bridge a foot each, the space between the two buses narrows considerably and I am only two and a half feet from both of them. Note that none of these measurements takes into consideration the outside rear-view mirrors. Six inches for my rear-view mirror and six inches for that of the bus or truck and now we are only a foot a half apart. Suppose one of those vehicles is a load king.

So, how close is too close? Perhaps some readers might want to get a couple of friends to park their vehicles separated by the distances mentioned above to see how comfortable they are in driving between them. If nothing else, it would be good practice.

• Steve Wolf is a Juneau resident.



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