diff --git a/ext/or-tools/routing.cpp b/ext/or-tools/routing.cpp
index c7b23e7..da1fa44 100644
--- a/ext/or-tools/routing.cpp
+++ b/ext/or-tools/routing.cpp
@@ -97,6 +97,25 @@ namespace Rice::detail {
   };
 } // namespace Rice::detail
 
+template<typename T>
+operations_research::Constraint *make_constraint(operations_research::Solver &solver, operations_research::IntExpr *left, T right, const std::string &op) {
+  if (op == "==") {
+    return solver.MakeEquality(left, right);
+  } else if (op == "!=") {
+    return solver.MakeNonEquality(left, right);
+  } else if (op == "<") {
+    return solver.MakeLess(left, right);
+  } else if (op == "<=") {
+    return solver.MakeLessOrEqual(left, right);
+  } else if (op == ">") {
+    return solver.MakeGreater(left, right);
+  } else if (op == ">=") {
+    return solver.MakeGreaterOrEqual(left, right);
+  } else {
+    throw std::runtime_error{"Unknown operator"};
+  }
+}
+
 void init_routing(Rice::Module& m) {
   auto rb_cRoutingSearchParameters = Rice::define_class_under<RoutingSearchParameters>(m, "RoutingSearchParameters");
   auto rb_cIntVar = Rice::define_class_under<operations_research::IntVar>(m, "RoutingIntVar");
@@ -293,24 +312,18 @@ void init_routing(Rice::Module& m) {
 
   Rice::define_class_under<operations_research::Solver>(m, "RoutingSolver")
     .define_method(
-      "add",
-      [](operations_research::Solver& self, Object o) {
-        operations_research::Constraint* constraint;
-        if (o.respond_to("left")) {
-          operations_research::IntExpr* left(Rice::detail::From_Ruby<operations_research::IntVar*>().convert(o.call("left")));
-          operations_research::IntExpr* right(Rice::detail::From_Ruby<operations_research::IntVar*>().convert(o.call("right")));
-          std::string op = o.call("op").to_s().str();
-          if (op == "==") {
-            constraint = self.MakeEquality(left, right);
-          } else if (op == "<=") {
-            constraint = self.MakeLessOrEqual(left, right);
-          } else {
-            throw std::runtime_error{"Unknown operator"};
-          }
+      "add_constraint",
+      [](operations_research::Solver& self, operations_research::Constraint& constraint) {
+        self.AddConstraint(&constraint);
+      })
+    .define_method(
+      "_make_constraint",
+      [](operations_research::Solver& self, Object left, Object right, Symbol op) {
+        if (right.class_of().name() == "ORTools::Constant") {
+          return make_constraint(self, Rice::detail::From_Ruby<operations_research::IntVar*>().convert(left), Rice::detail::From_Ruby<int64_t>().convert(right.call("value")), op.str());
         } else {
-          constraint = Rice::detail::From_Ruby<operations_research::Constraint*>().convert(o);
+          return make_constraint(self, Rice::detail::From_Ruby<operations_research::IntVar*>().convert(left), Rice::detail::From_Ruby<operations_research::IntVar*>().convert(right), op.str());
         }
-        self.AddConstraint(constraint);
       })
     .define_method(
       "fixed_duration_interval_var",
diff --git a/lib/or-tools.rb b/lib/or-tools.rb
index 4b9dac1..93c1380 100644
--- a/lib/or-tools.rb
+++ b/lib/or-tools.rb
@@ -29,6 +29,7 @@
 # routing
 require_relative "or_tools/routing_index_manager"
 require_relative "or_tools/routing_model"
+require_relative "or_tools/routing_solver"
 
 # higher level interfaces
 require_relative "or_tools/basic_scheduler"
diff --git a/lib/or_tools/routing_solver.rb b/lib/or_tools/routing_solver.rb
new file mode 100644
index 0000000..618c7f0
--- /dev/null
+++ b/lib/or_tools/routing_solver.rb
@@ -0,0 +1,14 @@
+module ORTools
+  class RoutingSolver
+    def add(comparison)
+      case comparison
+      when Comparison
+        add_constraint(_make_constraint(comparison.left, comparison.right, comparison.op))
+      when Constraint
+        add_constraint(comparison)
+      else
+        raise TypeError, "Not supported: RoutingSolver#add(#{comparison})"
+      end
+    end
+  end
+end
diff --git a/test/routing_constraints_test.rb b/test/routing_constraints_test.rb
new file mode 100644
index 0000000..bd3791c
--- /dev/null
+++ b/test/routing_constraints_test.rb
@@ -0,0 +1,112 @@
+require_relative "test_helper"
+
+class RoutingConstraintsTest < Minitest::Test
+  def test_no_extra_constraints
+    build_routing
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 2, 1, 0], route
+  end
+
+  def test_var_less_than_or_equal_var
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) <= @distance_dimension.cumul_var(@manager.node_to_index(2)))
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 1, 2, 0], route
+  end
+
+  def test_var_less_than_or_equal_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) <= 2455)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 1, 2, 0], route
+  end
+
+  def test_var_equal_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) == 2451)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 1, 2, 0], route
+  end
+
+  def test_var_equal_const_failure
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) == 2455)
+    solve
+
+    assert_equal :fail, @routing.status
+  end
+
+  def test_var_not_equal_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) != 2451)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 2, 1, 0], route
+  end
+
+  def test_var_less_than_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) < 2455)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 1, 2, 0], route
+  end
+
+  def test_var_greater_than_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) > 2455)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 2, 1, 0], route
+  end
+
+  def test_var_greater_than_or_equal_const
+    build_routing
+    @routing.solver.add(@distance_dimension.cumul_var(@manager.node_to_index(1)) >= 2455)
+    solve
+
+    assert_equal :success, @routing.status
+    assert_equal [0, 2, 1, 0], route
+  end
+
+  private
+
+  def build_routing
+    @manager = ORTools::RoutingIndexManager.new(3, 1, 0)
+    @routing = ORTools::RoutingModel.new(@manager)
+    transit_callback_index = @routing.register_transit_matrix([
+      [0, 2451, 731],
+      [2451, 0, 1745],
+      [731, 1745, 0],
+    ])
+    @routing.set_arc_cost_evaluator_of_all_vehicles(transit_callback_index)
+    @routing.add_dimension(transit_callback_index, 0, 10000, true, "Distance")
+    @distance_dimension = @routing.mutable_dimension("Distance")
+  end
+
+  def solve
+    @solution = @routing.solve(first_solution_strategy: :path_cheapest_arc)
+  end
+
+  def route
+    route = []
+    index = @routing.start(0)
+    while !@routing.end?(index)
+      route << @manager.index_to_node(index)
+      index = @solution.value(@routing.next_var(index))
+    end
+    route << @manager.index_to_node(index)
+    route
+  end
+end