The County may allow the widening of a driveway with limited sight
distance or may allow the relocation of a driveway with limited sight
distance to a location on the property frontage with better sight
distance. This will be allowed on routes with normal right-of-way,
provided there is no change in driveway usage. The following responsibility
clause must be added to the permit. "Applicant understands that the
existing sight distance for this driveway is less than current design
standards and the driveway modification, while beneficial to the property
owner, will not remedy the sight distance limitation."
Both vertical and horizontal alignment can limit sight distance.
In order to measure actual sight distance limited by vertical alignment,
place a sighting target four and one-quarter (4.25) feet above the
edge of pavement at a point twelve (12) feet from the edge of pavement
(approximate location of a driver approaching the roadway) at the
proposed driveway location. Sighting from a height of three and one-half
(3.5) feet, move along the roadway away from the proposed driveway
site to a point beyond where the target disappears. Now move toward
the target until it can first be seen and place a mark on the pavement.
Measure the distance along the roadway between the mark and the target.
Measurement may be made with an accurate measuring device mounted
on an automobile. This measured distance is the sight distance.
Even when the applicant is present, sight distance measurements
in terms of feet may be difficult for an applicant to understand when
it comes to getting on and off the roadway. A measurement of time
lapse may help the applicant get a better understanding of the critical
nature of the situation.
A sight distance visibility time for the driver exiting a driveway
to see an approaching vehicle can be used. A value of three (3) seconds
enables a stopped passenger car to cross a two-lane highway. A value
of seven (7) seconds allows vehicles exiting the driveway to turn
left or right onto two-lane roads without interference (slowing down)
of through traffic at speeds up to thirty-five (35) mph.
Trucks require greater sight distance than needed for passenger
cars, however, the greater driver eye height, typically over six (6)
feet, provides an allowance for vertical curve conditions. If the
obstruction to sight is a horizontal curve or other lateral blockage,
a fifty percent (50%) increase in visibility time is recommended.
Grading on the right-of-way to improve sight distance should
be considered and included in the permit for driveway construction.
Public street entrances should meet or exceed decision sight
distance.
Table 10. Sight Distance Requirements
Posted Speed: 35 mph.
Minimum Stopping Sight Distance: 250 feet.
Minimum Decision Sight Distance: 275 feet.
Source: The above distances are based on Exhibit 3-1 and 3-3
of the AASHTO Green Book 2004 Edition.