WAITEMATA TRAFFIC.

New Zealand Herald, Volume LXVI, Issue 20367, 23 September 1929, Page 14

WAITEMATA TRAFFIC.

BRIDGE OR TUNNEL.

AMERICAN EXPERIENCE

QUESTION OF CLEARANCE.

The following has boon received from Mr. D. E. Hiirkness, lecturer in civil engineering at the Auckland University College, in reference to criticisms by Mr. R. 11. Greville at a meeting of the Waitemata Chamber of Commerce of statements made by Mr. Harkness in nn address to a previous moeting of the chamber :

My remarks regarding tolls charged on American bridges applied to all the recent largo American bridges and vehicular tunnels that are self-supporting, and particularly to those of ban I'rancisco Bay. In this connection, I would repeat what was told me some little time ago by Mr. M. M. O'Shaughnessy, tho technical adviser to the San Francisco Board of Supervisors, which has under its control all the harbour structures al;ovo and below water in and around San Francisco. Speaking of the San Mateo bridge, Mr. O'Shaughnessy said: "The tolls on this bridge are 45 cents for each motor-car, plus 5 cents for each passenger. Other similar bridges have been built, some with higher tolls and some with lower tolls."

Ferry Rates as Base

Around Now York city tho situation is similar. Tho Port of Now York Authority is now building several very large bridges and is considering certain vehicular tunnels as well. The tolls to bo charged on these structures, like those on existing ones, will be based on tho existing ferry rates. In an exhaustive investigation of this subject, the Port of New York Authority found that "an average rate of 60 cents for vehicles, except for certain classes of through traffic, 5 cents for passengers in vehicles, 75 cents for buses, 5 cents for passengers in buses, and 5 cents for pedestrians, were reasonable charges upon which to base the estimates." In the Holland vehicular tunnels, N.Y., the charges for motor vehicles range from 50 cents up to 2 dollars. Ihoso charges are also based on tho existing ferry rates. Many other toll structures in other parts of tho country might also be mentioned where the charges are of like nature. It is of little uso therefore for Mr. Greville to declare that the United States toll charges are in reality very much less than stated, becauso those on the Vancouver Bridge are 15 cents and 5 conts. Vancouver is not in the United States, and it is generally admitted in Canada that prices and wages are lower in Vancouver than in other largo towns of that country. Prices and wages in tho large towns of the United States aro generally higher than in thoso of Canada. Vancouver toll charges cannot therefore be taken as truly representative, even of Canada.

Comparison o! Costs.

There exists also a misunderstanding of certain comparisons I made to tho Waitemata Chambor of Commerce as between the costs of bridges and tunnels. A few particular instances only were cited, and these were chosen because they afforded a uniques opportunity of comparing the costs of bridges and tunnels actually constructed at about the same time and place. Naturally such comparisons are not of general application, as the relative merits and costs of bridge and tunnel at any other place depend on the local conditions and requirements, and vary enormously from place to place. The instances mentioned included two cases of large bridges built across the Fast River at New York at about the same time as neighbouring tunnels of much smaller capacity. Tho actual costs of tho tunnels, consequently, were onethird and one-quarter respectively those of the bridges; tind their capacities about one-fifth and one-sixth respectively tho theoretical capacities of the bridges. It was pointed out that in these two instances, at any rato, taking all things into consideration, it would probably have been wiser and cheaper to construct smallcapacity tunnels as required rather than there huge Williamsburg and Manhattan bridges. If this had been done the enormous cost of rebuilding tho bridges to take the heavier modern traffic would have been obviated, money would not have been expended on facilities years before they could be utilised to tho full, and the existing traffic congestion around the bridge approaches would not bo cutting down tho actual capaciy of each structure to a fraction of its theoretical value. It is well known that these two bridges have never yet been running to capacity, and unless drastic remodelling of tho approaches is resorted to they never will. Efficient Vantilation.

The Holland twin vohicular tunnels under the Hudson lliver at New York have proved conclusively that mechanical ventilation of tunnels carrying motor traffic can be acomplishcd safely and efficiently. On tho first day of operation of this now traffic artery over 25,000 motor vehicles were passed through each of these two-track tunnols, the ventilating machinery was at no timo running anywhere near its gicatest capacity, and tho carbon-monoxide content at no time reached four parts per 10,000-the tunnel limit of carbon-monoxide impurity, which is constantly being exceeded in the streets of Now York. Although expensive methods had to bo adopted in tho construction of these tunnels they are paying handsomoly; while other and cheaper methods not yet used in New Zealand for subaqueous tunnel construction can in somo cases bo employed.

Finally, in regard to clearances of highlevel bridges over navigable waterways abroad, it should be explained that in the past the United States War Department has permitted a minimum clearance of 135 ft. It is generally conceded, however, that 150 ft. will be required in future. The engineering commission reporting on the Merspy crossing at Liverpool, after reviewing tho clearances of existing harbour bridges in other parts of the world, affirmed that if a bridge were built across the Mersoy it should have a clearance of at loast 185 ft. The colossal new Hudson Kiver bridge will increase this figure to 203 ft.

Of course, large ocean-going traffic does not at present require to use our upper harbour. But within the lifetime of any harbour bridge thfe situation might be very different indeed, and a structure put up to-day at great expense and with insufficient foresight might easily become a nuisance to future generations.