| Show U S disguises testing device to spy on automobile dr drivers vers A black rubber tube laid across the highway and connected at either end with a simple instrument staked at the side ol of the road forms the basis tor for analysis ot of how bow the motor public behaves in its use of the highways this special study is being made by the public roads bureau the tube is inconspicuous and looks much like an expansion joint one ot of its good features most motorists will not see the tube and will not be aware of making a record other devices such as checking by observers or by motion picture records are less lesi useful because they are likely to make drivers self conscious and cause them to slow down for fear of a speed trap A record of natural driving on a representative stretch of road is what is wanted the tubes are placed at 50 foot intervals on the stretch of road under I 1 study a halt half mile perhaps when an automobile crosses a tube a wave or puff of air Is forced into the roadside instrument and moves a diaphragm which causes an elec contact the tubes are disconnected at the middle of the road I 1 so that a vehicle passing on one side of the road does not register on the I 1 instrument ment on the oth other er side wires run to a recording dirig apparatus in which a strip of paper unrolls under a set of pens each passing car leaves a pair ot of pen strokes for front and rear wheels knowing the spacing of the tubes and the speed of the moving paper it is easy to calculate the speed of any car the device is particularly useful in studying the passing of slow vehicles by faster ones on a two lane highway the pen marked strip reveals the taster faster car overtaking the slower then swinging across the center and registering on both lanes then running in the left lane and aln finally ally swinging back to cotee the right distances and speeds aan can bi be calculated exactly and by tabulating many gassings pas sings it is ii possible to cori compute tor for example how long a straightaway stretch should be to make passing sam safe and easy A single tube with the roadside instruments connect connected to an electric counter makes a convenient and inexpensive device tor for registering the number of vehicles using a road |
