diff --git a/lib/google_maps_service/apis/directions.rb b/lib/google_maps_service/apis/directions.rb
index 6ae9fa2..2425082 100644
--- a/lib/google_maps_service/apis/directions.rb
+++ b/lib/google_maps_service/apis/directions.rb
@@ -57,13 +57,19 @@ module Directions
     #     `rail` is equivalent to `["train", "tram", "subway"]`.
     # @param [String] transit_routing_preference Specifies preferences for transit
     #     requests. Valid values are `less_walking` or `fewer_transfers`.
+    # @param [String] traffic_model Specifies the assumptions to use when calculating time in traffic.
+    #     This setting affects the value returned in the duration_in_traffic field in the response,
+    #     which contains the predicted time in traffic based on historical averages. The traffic_model
+    #     parameter may only be specified for driving directions where the request includes a
+    #     `departure_time`, and only if the request includes an API key or a Google Maps APIs Premium
+    #     Plan client ID. Valid values are `best_guess` (default), `pessimistic` or `optimistic`.
     #
     # @return [Array] Array of routes.
     def directions(origin, destination,
         mode: nil, waypoints: nil, alternatives: false, avoid: nil,
         language: nil, units: nil, region: nil, departure_time: nil,
         arrival_time: nil, optimize_waypoints: false, transit_mode: nil,
-        transit_routing_preference: nil)
+        transit_routing_preference: nil, traffic_model: nil)
 
       params = {
         origin: GoogleMapsService::Convert.waypoint(origin),
@@ -94,6 +100,11 @@ def directions(origin, destination,
 
       params[:transit_mode] = GoogleMapsService::Convert.join_list("|", transit_mode) if transit_mode
       params[:transit_routing_preference] = transit_routing_preference if transit_routing_preference
+      params[:traffic_model] = GoogleMapsService::Validator.traffic_model(traffic_model) if traffic_model
+
+      if traffic_model and not departure_time
+        raise ArgumentError, 'Must specify departure_time if specifying traffic_model'
+      end
 
       return get('/maps/api/directions/json', params)[:routes]
     end
diff --git a/lib/google_maps_service/apis/distance_matrix.rb b/lib/google_maps_service/apis/distance_matrix.rb
index 87f9bbb..c396586 100644
--- a/lib/google_maps_service/apis/distance_matrix.rb
+++ b/lib/google_maps_service/apis/distance_matrix.rb
@@ -52,13 +52,19 @@ module DistanceMatrix
     #     `rail` is equivalent to `["train", "tram", "subway"]`.
     # @param [String] transit_routing_preference Specifies preferences for transit
     #     requests. Valid values are `less_walking` or `fewer_transfers`.
+    # @param [String] traffic_model Specifies the assumptions to use when calculating time in traffic.
+    #     This setting affects the value returned in the duration_in_traffic field in the response,
+    #     which contains the predicted time in traffic based on historical averages. The traffic_model
+    #     parameter may only be specified for driving directions where the request includes a
+    #     `departure_time`, and only if the request includes an API key or a Google Maps APIs Premium
+    #     Plan client ID. Valid values are `best_guess` (default), `pessimistic` or `optimistic`.
     #
     # @return [Hash] Matrix of distances. Results are returned in rows, each row
     #     containing one origin paired with each destination.
     def distance_matrix(origins, destinations,
         mode: nil, language: nil, avoid: nil, units: nil,
         departure_time: nil, arrival_time: nil, transit_mode: nil,
-        transit_routing_preference: nil)
+        transit_routing_preference: nil, traffic_model: nil)
       params = {
         origins: GoogleMapsService::Convert.waypoints(origins),
         destinations: GoogleMapsService::Convert.waypoints(destinations)
@@ -78,6 +84,11 @@ def distance_matrix(origins, destinations,
 
       params[:transit_mode] = GoogleMapsService::Convert.join_list('|', transit_mode) if transit_mode
       params[:transit_routing_preference] = transit_routing_preference if transit_routing_preference
+      params[:traffic_model] = GoogleMapsService::Validator.traffic_model(traffic_model) if traffic_model
+
+      if traffic_model and not departure_time
+        raise ArgumentError, 'Must specify departure_time if specifying traffic_model'
+      end
 
       return get('/maps/api/distancematrix/json', params)
     end
diff --git a/lib/google_maps_service/validator.rb b/lib/google_maps_service/validator.rb
index 51c37fd..6231bf7 100644
--- a/lib/google_maps_service/validator.rb
+++ b/lib/google_maps_service/validator.rb
@@ -35,5 +35,19 @@ def avoid(avoid)
       end
       avoid
     end
+
+    # Validate traffic model. The valid values of traffic are `best_guess`, `pessimistic` or `optimistic`.
+    #
+    # @param [String, Symbol] traffic_model Traffic model to be validated
+    #
+    # @raise ArgumentError The traffic model is invalid.
+    #
+    # @return [String] Valid traffic model
+    def traffic_model(traffic_model)
+      unless [:best_guess, :pessimistic, :optimistic].include?(traffic_model.to_sym)
+        raise ArgumentError, 'Invalid traffic model.'
+      end
+      traffic_model
+    end
   end
 end
\ No newline at end of file
diff --git a/spec/google_maps_service/apis/directions_spec.rb b/spec/google_maps_service/apis/directions_spec.rb
index 7cebeb0..7a3091c 100644
--- a/spec/google_maps_service/apis/directions_spec.rb
+++ b/spec/google_maps_service/apis/directions_spec.rb
@@ -146,4 +146,18 @@
       expect(a_request(:get, 'https://maps.googleapis.com/maps/api/directions/json?origin=Sydney+Town+Hall&destination=Parramatta+Town+Hall&alternatives=true&key=%s' % api_key)).to have_been_made
     end
   end
+
+  context 'traffic_model parameter' do
+    it 'should call Google Maps Web Service' do
+      now = Time.now
+      client.directions('Sydney Town Hall', 'Parramatta Town Hall', traffic_model: 'pessimistic', departure_time: now)
+      expect(a_request(:get, 'https://maps.googleapis.com/maps/api/directions/json?origin=Sydney+Town+Hall&destination=Parramatta+Town+Hall&traffic_model=pessimistic&departure_time=%d&key=%s' % [now.to_i, api_key])).to have_been_made
+    end
+
+    it 'should error if departure_time not provided' do
+      expect {
+        client.directions('Sydney Town Hall', 'Parramatta Town Hall', traffic_model: 'pessimistic')
+      }.to raise_error ArgumentError
+    end
+  end
 end
\ No newline at end of file
diff --git a/spec/google_maps_service/apis/distance_matrix_spec.rb b/spec/google_maps_service/apis/distance_matrix_spec.rb
index ee15a80..7c01539 100644
--- a/spec/google_maps_service/apis/distance_matrix_spec.rb
+++ b/spec/google_maps_service/apis/distance_matrix_spec.rb
@@ -87,4 +87,22 @@
       }.to raise_error ArgumentError
     end
   end
+
+  context 'traffic_model parameter' do
+    it 'should call Google Maps Web Service' do
+      origins = ["Vancouver BC", "Seattle"]
+      destinations = ["San Francisco", "Victoria BC"]
+      now = Time.now
+      client.distance_matrix(origins, destinations, traffic_model: 'pessimistic', departure_time: now)
+      expect(a_request(:get, 'https://maps.googleapis.com/maps/api/distancematrix/json?origins=Vancouver+BC%%7CSeattle&destinations=San+Francisco%%7CVictoria+BC&traffic_model=pessimistic&departure_time=%d&key=%s' % [now.to_i, api_key])).to have_been_made
+    end
+
+    it 'should error if departure_time not provided' do
+      expect {
+        origins = ["Vancouver BC", "Seattle"]
+        destinations = ["San Francisco", "Victoria BC"]
+        client.distance_matrix(origins, destinations, traffic_model: 'pessimistic')
+      }.to raise_error ArgumentError
+    end
+  end
 end
\ No newline at end of file
diff --git a/spec/google_maps_service/validator_spec.rb b/spec/google_maps_service/validator_spec.rb
index 72aaa99..95cc775 100644
--- a/spec/google_maps_service/validator_spec.rb
+++ b/spec/google_maps_service/validator_spec.rb
@@ -33,10 +33,29 @@
       end
     end
 
-    context 'with invalid travel mode' do
+    context 'with invalid route restriction' do
       it 'should raise ArgumentError' do
         expect { GoogleMapsService::Validator.avoid('people') }.to raise_error ArgumentError
       end
     end
   end
+
+  describe '.traffic_model' do
+    context 'with valid traffic model' do
+      it 'should return the traffic model' do
+        [:best_guess, :pessimistic, :optimistic].each do |mode|
+          expect(GoogleMapsService::Validator.traffic_model(mode)).to eq(mode)
+        end
+        ['best_guess', 'pessimistic', 'optimistic'].each do |mode|
+          expect(GoogleMapsService::Validator.traffic_model(mode)).to eq(mode)
+        end
+      end
+    end
+
+    context 'with invalid traffic model' do
+      it 'should raise ArgumentError' do
+        expect { GoogleMapsService::Validator.traffic_model('nonexistent') }.to raise_error ArgumentError
+      end
+    end
+  end
 end
\ No newline at end of file