Inadequate drainage causes both driving problems and rapid deterioration of
roads.  Pavement drainage requires consideration of surface drainage, gutter flow, and
inlet capacity.  These elements depend on storm frequency and the allowable spread of
storm water on the pavement surface.

 Several decisions are involved in selecting the recurrence interval and spread.
      1. The classification of the highway defines the public’s expectations
      regarding water on the pavement surface.  Ponding on traffic lanes of high
      speed, high volume highways is contrary to the publics expectations.
      2. Design speed is important to the selection of design criteria.  At speeds
      greater than 40 mph, water on the pavement can cause hydroplaning.
      3. With higher traffic volume, projected costs of traffic delays and
      accidents are also higher.
      4. The intensity of rainfall can significantly affect the selection of design
      frequency and spread.
      5. Cost considerations make it necessary to formulate a rational decision
      matrix.

     Elevation of the highway and surrounding terrain must also be considered when
water can be drained only through a storm drainage system.  Spread on traffic lanes can be
tolerated to greater widths where traffic volumes and speeds are low.

     A check storm should be used anytime runoff could cause unacceptable flooding.
Inlets should be evaluated when a series of inlets terminate at a sag vertical curve, where
ponding could occur.  Criteria for spread during the check storm are:

      1. One lane open to traffic during the check storm event.
      2. One lane free of water during the check storm event.

     The potential for hydroplaning can be reduced in a number of ways.
 
      1. Design highways to reduce drainage path lengths of water flowing over
      the pavement to reduce flow build-up.
      2. An increase of the pavement surface texture, such as grooving or
      grinding, will increase the drainage capacity.
      3. The use of open-graded asphaltic pavements greatly reduce the
      hydroplaning potential of the roadway.
      4. Drainage structures to capture the flow of water over the pavement will
      reduce the hydroplaning potential.

     The recommended minimum values of roadway longitudinal slope given in the
AASHTO Policy on Geometric Design, includes these guidelines:
 
      1. A minimum longitudinal gradient is more important for a curbed than for
      an uncurbed pavement since water is constrained by the curb.  Flat
      gradients on an uncurbed surface can lead to a spread problem if vegetation
      builds up along the pavement edge.
      2. Gutter grades should not be less than .5% for a curbed pavement with an
      absolute minimum of .3%.  These can be obtained by using a rolling profile,
      or by warping the cross slope.
      3. To provide adequate drainage in sag vertical curves a minimum slope of
      .3    % should be maintained within 50 ft of the low point of the curve.

     A cross slope of 2% has little effect on driver effort in steering.  In areas of intense
rainfall 2.5% may be used to facilitate drainage.  On multi-lane highways sloped in the
same direction the two adjacent to he crown line should be pitched at the normal slopes
and successive lane pairs should be increased by .5% to 1.0%.  The maximum pavement
cross slope should be 4.0%

     Curbs are used at the outside edge of pavements for low speed highways and in
some case adjacent to moderate to high-speed highways.  They serve the following
purposes.
      -Contain the surface runoff.
      -Prevent erosion on fill slopes.
      -Provide pavement declination.
      -Enable development of property next to the road.

     Gutters in combination with curbs can come in 12in - 39in widths.  Gutter cross
slopes can be the same as the road or steeper.  AASHTO states that 8% slope is a
common maximum cross slope for gutters.

     Roadside channels are commonly used with uncurbed roadway sections.  They can
be used in cut sections, depressed sections and other areas where sufficient right of way is
available and driveways are limited.  To prevent drainage from median area across travel
lanes, slope median areas and inside shoulders to a center swale.

     Bridge deck drainage is a similar to that of curbed roadway sections and is
important for the following reasons.

      -Deck structure and reinforcing steel is susceptible to corrosion from
      deicing salts.
      -Moisture on bridge decks freezes before surface roadways.
      -Hydroplaning often occurs at shallower depths on bridges due to reduced
      surface texture of concrete bridge decks.

Bridge deck drainage is less efficient than roadway surfaces because cross slopes are
flatter, parapets collect large amounts of debris, and bridge scuppers are more easily
clogged.  Because of this, gutter flow should be intercepted before the bridge.  Also zero
gradients and sag verticals should be avoided on bridges.  Bridge runoff should also be
collected immediately after it flows onto the subsequent roadway.

     Slope shoulder areas adjacent to median barriers to the center to prevent drainage
from running across traffic lanes.  Sometimes it could be necessary to provide inlets or
slatted drains to collect the water accumulated against the median barrier.  Piping could
also be used.

Summarized by Kristopher Hamilton from the article in Better Roads, December 1997